Electromagnetic radiation - introduction to EM waves
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Doc Brown's school physics revision notes: GCSE
physics, IGCSE physics, O level physics, ~US grades 8, 9 and 10
school science courses or equivalent for ~14-16 year old students of
physics
Types, properties,
uses and the spectrum of visible light
This page will answer many questions about
the electromagnetic spectrum e.g. Be able to understand that all
electromagnetic waves are transverse and that they travel at the same speed
in a vacuum. Be able to understand that the
electromagnetic spectrum is continuous from radio waves to gamma rays, but
the radiations within it can be grouped in order of decreasing wavelength
and increasing frequency. Be able to describe the properties
and uses of the continuous
electromagnetic spectrum including in order
Sub-index for this page:
(a)
Some general points
on electromagnetic radiation
(b)
Examples of wave equation calculations
involving electromagnetic radiation
All the properties, uses
and dangers of
electromagnetic radiations are described in detail in individual sections
(c)
radio waves *
(d)
microwaves * (e)
infrared radiation
(f)
visible light
* (g)
ultraviolet
light * (h)
X-rays *
(i) gamma radiation
and the trend ==> increasing frequency, decreasing wavelength,
increasing energy of photons
(visible light includes the colours of the visible spectrum
- red, orange, yellow, green, blue, indigo, violet)
(j)
Excitation of atoms,
ionisation and more on the dangers of ionisation radiation
(k)
Some general learning
objectives
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(a) Some general points
on electromagnetic radiation
we are talking about ...
radio waves, microwaves,
infrared radiation (ir), visible light spectrum,
ultraviolet light (uv), X-rays,
gamma radiation
Waves for communication and to provide evidence that the
universe is expanding
Electromagnetic (EM) radiations are an
example of transverse waves - the oscillations are at 90o
to the direction of travel..
Unlike longitudinal sound waves, which
need a material for the vibrations to pass through,
electromagnetic radiation can pass through a vacuum.
But, note that many materials are partially or wholly
transparent to various electromagnetic radiations.
All EM radiations are oscillating
(vibrating) electric and
magnetic fields.
The energy is transferred
in little packets of energy called photons.
All the types of EM radiation travel at the
same speed in vacuum ('empty space'), with little
differentiation in air, which has a very low density.
The speed of light
in vacuum is 3.0 x 108 m/s.
However, in passing into transparent dense
materials like glass or water the speed is significantly reduced and this reduction
depends on the wavelength (see
Refraction and the visible light spectrum - prism
investigations notes).
EM radiations have a huge
variety of wavelengths and corresponding frequencies, whose
ranges are quoted in the 2nd table below including the trends
from radio waves to gamma radiation.
The seven 'types' of
radiation are primarily grouped on the basis of their individual
properties and effects - which can be quite different because of
the difference in energy carried by the EM radiation.
The higher the frequency of
the EM radiation, the shorter the wavelength and the greater the
energy transferred.
We, and other animals and
plants, can detect some of the EM radiations e.g. our skin
detects the infrared heat radiation from the Sun, our eyes
detect the colours of the visible spectrum and many insects can
detect uv radiation.
All EM radiations are emitted from a source and spread out in all possible
directions.
The EM waves will continue to travel through any
medium until they are absorbed - which may be just a surface
itself, or partially absorbed or most passes through
e.g. visible light through colourless glass.
The sources of EM radiation
are even more varied than the seven types, but they all involve
energy changes of all atoms, all molecules and even the nucleus
of some atoms.
All sources are described in the individual
sections.
When any type of EM radiation
is absorbed by a material it is no longer energy in EM wave
form.
The production and
absorption of radiation involves energy store exchanges
The EM radiation is converted into another form of energy.
Much of it eventually ends up as heat - increasing the thermal
energy store of the absorbing material or surroundings.
The dissipation of
energy is mainly due to the radiation spreading out
in all directions from the source.
This also means that
signals e.g. radio waves, become much weaker the further
from the transmission source.
One exception to
this, are laser beams, which can be highly focussed into
a narrow coherent beam.
Three higher energy
EM radiations (uv, X-rays and gamma) can initially cause ionisation - the process of knocking off outer electrons of
atoms to create positive ions (see
Appendix 1 for more details). That is why these three
are referred to as ionising radiations.
A lot of the new technology
in industry, medicine and university research that developed through the
20th century and on into the 21st century involves the use of
all types of EM
radiation.
This is not without
issues that must be resolved.
The dangers from EM
radiation often depend on how much energy they carry.
For increase in
energy and danger its usually
gamma > X-ray > uv > visible > infrared > microwave > radio
The risks and benefits of
any new technology must be carefully evaluated and the use
of such technologies must safe and carefully regulated.
that is not to say
certain technologies should be banned, but risks must be
assessed and appropriate safeguards put in place.
In each of the 7 sections
I've described the properties and uses of EM radiations.
When waves meet a
boundary they can be absorbed, reflected, refracted or
transmitted. What happens depends on the properties of
the wave and the nature of the boundary. The result of
the boundary - EM radiation interaction can be used to
investigate things you cannot see by other methods.
Gamma ray,
infrared and X-ray EM radiations are all used in
medical imaging techniques to help doctors
diagnose medical conditions.
Ultrasound is
also used, but this is NOT an EM radiation.
The techniques
used in medical imaging are potentially dangerous,
so to use them, you have to compromise and hopefully
get a good quality image without harming the
patient.
I've also
described possible hazards and how they are
minimised, but also examples of where danger to life is
balanced against trying to save life - a good example is
cancer treatment using gamma radiation and the diagnostic
uses of X-rays.
Do we need notes on how
Herschel and Ritter contributed to the discovery of waves outside the limits of
the visible spectrum, namely ir and uv?
Four familiar 'parts' of the electromagnetic spectrum and the
wave equation
The higher the
frequency, the shorter the wavelength.
You should know these trends and the order
with respect to seven electromagnetic radiations
Electromagnetic radiation |
Radio waves
TV and radio |
Microwaves
cooking |
Infrared radiation
heat radiation |
Visible light
eye - vision |
Energy |
========= increasing
energy of radiation ======> |
Frequency |
=== increasing frequency
of radiation (Hz) ======> |
Wavelength |
====== decreasing
wavelength of radiation (m) ======> |
'picture trend' |
 |
 |
 |
 |
These four EM radiations are all part of daily life!
The radio waves can be split into:
long-wave radio (LW),
medium-wave radio (MW), short-wave radio (SW),
very high frequency radio (VHF) and ultra high
frequency radio (UHF)
A
greater range of the electromagnetic spectrum - seven
varieties grouped by their wavelength and frequency
Type of electromagnetic radiation
===> |
Radio waves |
Microwaves |
Infrared radiation |
Visible light |
Ultraviolet light |
X-rays |
Gamma rays |
~wavelength range/m |
>10-1 |
10-4 to 10-1 |
7 x 10-7
to 10-4 |
4 x 10-7
to 7 x 10-7 |
10-8 to
4 x 10-7 |
10-9 to 10-8 |
< 1 x 10-9 |
~typical wavelength/m |
103 |
10-2 |
10-5 |
5 x 10-7 |
10-8 |
10-9 |
<10-12 |
~wavelength range/nm |
> 108 |
105 - 108 |
700 - 105 |
400 - 700 |
10 - 400 |
1-10 |
< 1 |
~frequency range/Hz |
< 3 x 109 |
3 x 109 to
3 x 1012 |
3 x 1012 to
4.3 x 1014 |
4.3 x 1014 to
7.5 x 1014 |
7.5 x 1014
to 3 x 1016 |
3 x 1016
to 3 x 1017 |
> 3 x 1017 |
Photon energy trend |
=========== increasing
energy of radiation photons ========> |
Frequency trend |
=========== increasing frequency
of radiation (Hz) =========> |
Wavelength trend |
========== decreasing
wavelength of radiation (m) =========> |
'picture trend' !! |
================================> |
Ultraviolet light, X-rays and
gamma rays would not normally be part of daily life!
From left to right: same speed, increasing
frequency, decreasing wavelength and increasing energy of
photon.
Our senses can only
detect specific regions of the spectrum e.g. eyes detect
visible light and its different colours and our skin detects
infrared radiation as 'heat'.
EM radiation always transfers energy from
an emitter source to an absorber.
Think about ..
a radio
transmitter and receiver
an electric heater uses infrared
radiation to transfer energy from the hot element
thermal energy store to the thermal energy store of
the room and you!
The energy of the electromagnetic radiation photons is
directly proportional to their frequency.
This has particular
relevance for health and safety issues because generally
speaking the electromagnetic radiation is most dangerous
from the three on the right.
Remember that when EM waves meet a
boundary they may be absorbed, reflected, refracted or
transmitted and sometimes several these of effects at
the same time!
What happens at a boundary depends on the materials at the
boundary and the wavelength of the electromagnetic radiation
(i.e. the type of EM radiation).
All the properties and uses of
electromagnetic radiations are described in detail in individual sections
index:
radio waves *
microwaves
*
infrared radiation *
visible light
* ultraviolet
light *
X-rays *
gamma radiation
TOP OF PAGE and
full sub-index
(b)
Examples of wave
equation calculations involving electromagnetic radiation
You should know that all waves obey the
same wave
equation:
v = λ x f where
v is speed in metres per
second, m/s,
for electromagnetic radiation
calculations, the speed in vacuum/air is taken as
3.0 x 108 m/s
λ is wavelength in
metres, m
f is frequency in hertz,
Hz (per sec)
wave equation rearrangements:
f = v / λ
and
λ = v / f
f = v / λ and
λ = v / f
Note that you are not required to recall the value of the speed of
electromagnetic waves through a vacuum ...
.. it is very big, 'speed of light',
v,
is taken to be exactly
3.0 x 108 m/s AND to
be used in all questions
Be able to do examples of
calculations using the wave speed formula and its rearrangements.
speed (m/s) = wavelength (m) x
frequency (Hz, s-1)
v = f x λ
*
f = v / λ *
λ = v / f
You may also need:
speed (m/s) = distance (m) /
time (s)
period (s) = 1 / f (Hz)
Q1
A red light beam has a wavelength of 650 nm.
Calculate the frequency of red light
photons.
v = f x λ * f = v / λ
= 3 x 108 / (658 x 10-9) =
4.62 x
1014
Hz
Q2
A green laser light beam has a fixed frequency of 5.90 x 1014 Hz
Calculate the wavelength of the laser
light in m and nm.
v = f x λ *
λ = v / f = 3 x 108 / (5.90 x 1014) =
5.08 x
10-7
m
1 nm = 10-9 m, so wavelength =
5.08 x 10-7 x 109 =
508 nm
Q3
The time period of a electromagnetic radio wave is 5.0 x 10-5 seconds
(a) What is the frequency of the radio
wave?
period = 1 / frequency
f = 1 / period = 1 /
(5.0 x 10-5) =
2.0
x 104
Hz
(b) If the 'speed of light' is 3.0 x 108
m/s, calculate the wavelength of the radio wave in m.
λ
= v ÷ f = 3.0 x 108 / 2.0 x 104
= 1.5 x 104
m
(c) If the distance from the transmitting radio
station to your radio is 200 km, how long does it take the signal to reach
you?
The speed formula is v = d / t,
so t = d / v
= (200 x 1000) / (3.0 x 108) =
6.7 x 10-4
s
Q4
A wave of electromagnetic radiation has a wavelength of 25 cm
Calculate the frequency of the radiation.
25 cm = 25 / 100 = 0.25 m
v = f x λ *
f = v / λ =
3 x 108 / 0.25 =
1.2 x 109
Hz
Q5
A scanning device sends out a pulse of microwave radiation that takes15
µs to detect an echo.
How
far away is the reflecting object? (you should know that 1
µs = 10-6 s)
v =
d / t, so d = v x t = (3 x 108) x (15 x 10-6) = 4500 m
BUT, this the total distance 'there and back', so the reflecting object is
4500 / 2 = 2250 m
away.
Q6
A radio station is broadcasting its programmes with a radio frequency of 104
MHz.
Calculate the wavelength of the radio
signal. (you should know 1 MHz = 106 Hz)
v = f x λ, so λ = v / f = (3 x 108)
/ (104 x 106) =
2.89
m (3 sf)
Q7
For radio astronomy, the shortest wavelength of radio waves that can pass
through the Earth's atmosphere is 100 m.
Calculate the maximum frequency of radio
waves that can be detected.
v = f x λ * f = v / λ
= 3 x 108 / 100 =
3 x
106
Hz
Q8
A satellite is 75 km above the Earth's surface. (speed of light 3.00 x 108
m/s)
What is the shortest time that a
microwave signal would take to reach the satellite from the Earth's surface?
s = d / t, t = d / s = (75 x 1000)
/ (3.00 x 108) =
2.50 x
10-4
seconds
Q9
A red light wave has a wavelength of 7.0 x 10-7 m. (speed of light
3.00 x 108 m/s)
What is the frequency of the red light wave?
v = f x λ * f = v / λ
= 3 x 108 / (7.0 x 10-7 m) =
4.29 x
1014
Hz (3 sf)
Need Q using kHz, X-rays, gamma etc.
index:
radio waves *
microwaves
*
infrared radiation *
visible light
* ultraviolet
light *
X-rays *
gamma radiation
TOP OF PAGE and
full sub-index
(c) The properties and uses of
radio waves
– television and radio
for broadcasting and communication
The sources and properties of radio
waves
Making radio waves - transmitter: Radio
waves, like all EM radiations are oscillating electric and
magnetic fields, and are produced by alternating electric
currents (a.c.).
Alternating currents automatically produce an oscillating
electric and magnetic field that makes the electric
charges (electrons) oscillate and this automatically
produces radio waves if the oscillations of that frequency.
The frequencies of the waves produced
will equal the frequencies of the alternating current - however complex
the signal transmitted.
The diagram below illustrates the
principles of how radio signals are generated, transmitted and received.
The oscillating charges produces and emit an EM wave
of radiation - radio waves. The EM wave has the same frequency
as the a.c. current that produced it - so a radio transmitter circuit uses
a.c. frequencies in the EM radiation band to create
radio waves of the desired frequencies.
Receiving radio waves: EM waves can cause charged
particles like electrons to oscillate at the same frequency. If these
electrons are part of a 'receiver' electrical circuit, an alternating current is
induced at the same frequency as the EM wave - this is what a radio receiver
aerial (antenna) does and the rest of the electronics produces the
sound and pictures.
You can demonstrate by feeding a
signal from a microphone or signal generator into an oscilloscope and
into a radio transmitter circuit.
The signal from the radio
receiver can then be fed into a 2nd oscilloscope.
You can then compare the
generated signal with the received signal and they should be the
same, however complex the signal may be.
In real radio
transmission-reception situations
the complex
signal
frequencies use a carrier
wave.
A carrier
wave is
a pure wave of
constant frequency, a bit like a sine wave
and has imposed on it the more complex wave of the signal
information.
The process of imposing an input signal onto a carrier
wave is
called modulation.
The above descriptions about radio waves apply to microwaves too,
The combination of transmitter
(e.g. radio mast) and receiver (e.g aerial) allows you to
encode information onto a radio signal and transmit information from one
place to another - its essentially a data transfer system which might be
a radio signal or TV .
Radio waves do penetrate liquids and
solids but are gradually absorbed and end up as heat energy - increasing the
thermal energy store of the surroundings.
Radio waves pass straight through the
human body without causing harm.
Uses of radio waves
Radio waves are EM radiation with
wavelengths greater than 0.1 m.
Long-wave radio waves can be transmitted
over long distances, in fact all the way around the Earth. They have long
wavelengths of 1-10 km and can diffract around the surface of the Earth and
around hills too and reflect/refract off the ionosphere - see diagram and
comments below.
This means radio signals can be
received even if the transmitter and receiver are not in direct line of
sight.
Reflection/refraction of radio waves
Short-wave radio signals of wavelength
~10 m to ~104 m can also be transmitted and received over long distances
because they are reflected off the ionosphere (which is an electrically
charged layer of the Earth's upper atmosphere).
The lower atmosphere is not
electrically charged and does not inhibit the transmission of radio
waves or microwaves.
However, part of the upper
atmosphere, called the ionosphere, contains electrically charged
particles (ions) that interfere with radio waves e.g. reflect them,
and so they zig-zag through the lower atmosphere travelling over
thousands of km from transmitter to receiver.
Radio waves are used to transmit
information from one
location to another e.g. to your TV and radio 'appliances' for
you to view and/or listen to.
TV and FM radio use very short radio
waves and other radio transmissions use medium and longer wavelengths (MW
and LW). To get a good reception, you do need to be in direct sight of the
transmitter because these waves do not diffract (bend) around obstacles and
they don't travel through buildings.
Bluetooth devices use low
power short wavelength radio waves to
communicate and send data between devices like mobile phones or computers relatively close together - so the
radio waves only have to travel short distances. The distances between
devices are quite short i.e. within a few metres of each other.
This avoids the use of wires e.g. the
wireless headset you use to use your phone while driving a car.
The
diffraction of radio waves
You can pick up a long
wavelength (lower frequency long wave) radio signal with your radio receiver
without being in a direct line with the radio transmitter because these
radio waves are diffracted by hills and other large obstacles.
The wavelengths of 'long wave' radio
waves can be over a km, which is similar to the width of hills.
You can also pick up long wave
radio signals over large distances because they can diffract around the
Earth's surface.
For a good reception of higher
frequency (shorter wavelength, 'short wave') TV and FM radio signals, you
need to be in direct line with the transmitter, unlike 'long wave radio',
the signal shows little diffraction ('bending').
Short wavelength, high frequency
radio signals, can also be received over large distances because these
signals can bounce of the Earth's surface AND the ionosphere - an
electrically charged layer of the Earth's upper atmosphere - the waves
effectively zig-zag between the Earth's surface and the ionosphere (==
/\/\/\ ==>).
Unlike microwaves, radio waves are
refracted by some layers of the atmosphere, so cannot be used for satellite
communications.
You can now use high-frequency radio wave
scanners to security checks on passengers and luggage at airports.
Radio waves are much safer than using X-rays.
Dangers of radio waves?
I don't know of any? Does political
propaganda count?!!! (just a joke!)
Radio waves are of long wavelength
and low frequencies and so carry little energy and pass through our body
without doing any harm.
index:
radio waves *
microwaves
*
infrared radiation *
visible light
* ultraviolet
light *
X-rays *
gamma radiation
TOP OF PAGE and
full sub-index
(d)
The properties, uses
and dangers
of microwaves
–
microwave radiation for mobile phones and satellite television
communication
The sources and properties of
microwave radiation
Microwaves, like radio waves, are produced by an oscillating
electric and magnetic field in an electric circuit (no details required at GCSE level).
The explanation of how microwave
transmissions work is the same as for
radio waves
(so need to repeat the notes here)
Uses of microwave radiation
TV transmitters/receiver sets
use microwave signals via satellite communication ('satellite TV dishes').
Mobile phones ('satellite/cell
phone') can use satellite communication systems too, as well as local
transmitting/receiving 'mobile phone' masts that are now appearing
everywhere!
Microwave mobile phone call
signals are usually picked by, or transmitted by, the nearest 'mobile phone mast'
receiver/transmitter.
The path between the microwave transmitter or receiver
usually needs to be a straight line with no large buildings or other
obstruction blocking the signal. That's why you see so many tall mobile
phone relay masts are all over the place!
For some applications, the microwave signal must have a
wavelength to allow it to penetrate clouds (water droplets) and water vapour
and through the ionosphere to communicate
with satellites in orbit thousands of km above the Earth's surface.
Reflection/refraction of radio waves
Radio waves are refracted by some
layers of the atmosphere, so cannot be used for satellite
communications. However, microwaves of shorter wavelength, are not
refracted in the atmosphere and can be used for satellite
communications.
The lower atmosphere is not
electrically charged and does not inhibit the transmission of radio
waves or microwaves.
However, part of the upper
atmosphere, called the ionosphere, contains electrically charged
particles (ions) that interfere with radio waves, but not microwave
radiation.
The longer wavelength radio waves
are either reflected or refracted by the ionosphere, but the
microwaves can pass through the upper atmosphere to satellites orbiting high above the
Earth's surface.
The microwave signal (telephone
or TV) is transmitted through the Earth's atmosphere into space where the
satellite receiver dish (many miles above Earth's surface) picks up the signal.
The signal is then re-transmitted
back to a receiver on the Earth's surface eg TV satellite dish and detector.
e.g. telephone signals between
the UK and USA can be sent by microwaves via satellites.
There is a small time delay
between the transmission and reception of the signal because of the
long distance it travels and the operation of the electronics.
We
can detect microwave radiation from around the Universe using huge radio
telescope dishes - so microwaves are used by radio-astronomers to
investigate objects in the universe.
Satellite microwave
transmission-reflection-receiving can be used to monitor certain geophysical
aspects of the Earth's surface eg rainforest versus deforestation, ice sheet
cover and icebergs in arctic areas.
It has already pointed out that is
important for communications that the microwave frequency waves can pass
through any moisture in the Earth's atmosphere. However, its different
for cooking, where you want the opposite effect.
Microwave oven cookers use
microwave EM radiation to
heat up food and pretty rapidly as the high frequency microwaves readily
penetrates the food.
The microwave frequency used in cookers is slightly
different to that used in mobile phones and is absorbed by water and fat
molecules e.g. in food.
The microwaves can penetrate right
into the food several cm as it is absorbed by water molecules.
The water and fat molecules become
excited, increasing their kinetic energy of vibration.
On 'relaxing' to their normal
energy level, the molecules rapidly transfer this excess energy to
the food, increasing
its thermal kinetic energy store and rapidly cooking it.
The thermal energy is further
dispersed throughout the food by conduction.
If your skin is exposed to too much
microwave radiation you can damage cells and suffer burns, which is why
microwave cookers cannot operate with the door open!
Technically, the high frequency
radiation is absorbed by water molecules which spin round faster than
normal. When the water molecules 'relax' and return to their normal
energy state, the energy is transferred and released as heat to whatever
you are cooking. The heat energy of the water is distributed throughout
the food increasing the thermal energy store of your pizza!
Cooking note comparing use of
microwaves and infrared:
When food is grilled, initially
only the surface is cooked, because infrared is not very penetrating
and deeper inside the food will be less cooked - perhaps not
sufficiently for health and safety. However, in microwave cooking,
the radiation can penetrate deep into the food and quite quickly too
(often just a few minutes), so the food is more thoroughly cooked,
but not necessarily as tasty, since we like 'fried' food.
Microwave radar scanners
Microwave radar emission and
detection systems can be used as echolocation detectors e.g.
(i) Military application:
Microwave pulses can be sent out by a transmitter mounted on one
aircraft and monitoring the reflected echo to detect another
aircraft. The computer will work out the range and direction of
target.
(ii) Civilian application:
Airliners use microwave radar to detect a hazard e.g. another
aircraft that is too close
(iii) Microwave scanning is used
by meteorologists to monitor precipitation of rain or snow to help
with short-term weather forecasting.
Example of simple calculation (speed
of electromagnetic radiation)
Q Suppose an airliner
sends out a microwave radar signal of wavelength of 1.20 cm.
The microwave reflects off
another aircraft and the echo is detected after a time lapse of 6.0
µs.
The speed
of electromagnetic radiation = 3.00 x 108 m/s.
(a) What is
the frequency of the microwave beam?
speed =
wavelength x frequency
f = v ÷ λ =
3.00 x 108 ÷ (1.20 / 100) =
2.50 x 1010
Hz
(b) What is
the distance between the two aircraft?
s = d /
t, d = s x t = 3.00 x 108 x 6.0 x 10-6
= 1800 m (total distance including echo)
distance between aircraft = 1800 ÷
2 = 900 m
(µ
is micro = 10-6, and total distance is halved because
it involves 'there and back')
Dangers of microwave radiation
Direct exposure to high intensity
microwave radiation can cause burns - it has a similar effect to infrared
radiation. It can also damage your eyes - which is why microwave ovens can
only work with the door closed.
The inside metal walls of a microwave
cooker reflect the microwaves around and stop them exiting the cooker.
The door is fitted with special glass
that also reflect microwaves, so the cooker is designed to internally
retain any potentially harmful microwave radiation.
Dangers of using mobile phones?
There has been some controversy about the use of mobile phones.
Water
molecules readily absorb certain microwave frequencies and become heated
(this is how a microwave cooker works!).
So, potentially, since you
contain a lot of water, microwaves heat you up by being near a mobile phone mast or
excessive use of your mobile phone BUT there is no real evidence (as far as
I know?) to support the notion that there is a danger to you.
However, there are strict limits on
the amount of energy a mobile can transfer when emitting a signal.
In the UK the legal limit is 2
watts, that is 2 joules of energy per second.
This should cause no harm and it
is distributed in ALL directions.
index:
radio waves *
microwaves
*
infrared radiation *
visible light
* ultraviolet
light *
X-rays *
gamma radiation
TOP OF PAGE and
full sub-index
(e) The properties, uses
and dangers of
infrared waves (IR radiation)
See also
Introduction to heat transfer - including infrared radiation
and
Absorption
& emission of radiation by
materials - temperature & surface factors including infrared
The sources and properties of infrared
radiation (thermal radiation)
To obtain a viable source of FM infrared radiation ('heat
radiation') all you need is an energy store at a higher temperature than the
background e.g. hot water radiator, electric heater
The Sun is the most powerful emitter of
infrared radiation (thermal radiation) - energy is being continuously
released by nuclear fusion of hydrogen to helium, so it isn't cooling down!
When you heat up
materials the bonds between the atoms in the molecules vibrate more
energetically, and so the molecules are more 'energetic' with respect to the cooler
less 'vibrating' background molecules.
When the vibrations decrease as the
particles 'relax' to their normal energy levels, the energy
is released by the material emitting FM infrared radiation.
(There is also heat transfer by
conduction to any cooler material in contact with the hotter material.)
Infrared radiation is absorbed directly by molecules
- increasing their kinetic energy of movement/vibration and so increasing
the energy store of the absorbing material.
All materials are continually emitting and absorbing
infrared radiation and the hotter the material the more infrared radiation
it emits.
Uses of infrared radiation
An electric heater energy
store transfers and emits infrared radiation to warm you up and increase your thermal energy
store.
The
hot surface of radiators emit infrared radiation (but there is also
conduction from the hot water or electrical heating element to the surface,
so heat is also conveyed away by convection currents in the air).
All 'hot
heaters' transmit infrared EM radiation to increase the thermal energy store
of the surroundings increasing its temperature.
When you grill food e.g. toasting bread,
you are using infrared radiation to raise the temperature of the food - the
surface of the food absorbs the radiant energy from the toaster's heating
elements, from one thermal energy store to another.
 |
 |
The electrical resistance elements of a cooker
ring or a toaster become hot enough to emit a strong beam of infrared
radiation to heat the contents of a pan (left) or grill the toast
(right).
Electrical energy is converted into
thermal energy which increases the
thermal energy store of the heating elements, some of which is converted
to infrared
radiation, which on absorption, increases the thermal
energy store of the pan and contents or bread being toasted etc.
Note on cooking techniques
(i) When food is grilled,
initially only the surface is cooked, because infrared is not very
penetrating and deeper inside the food will be less cooked - perhaps
not sufficiently for health and safety.
(ii) In microwave cooking, the
radiation can penetrate deep into the food and quite quickly too
(often just a few minutes), so the food is more thoroughly cooked,
but not necessarily as tasty, since we like 'fried' food.
(iii) When food is cooked in an
oven, plenty of time is usually allowed for the heat to conduct
right through the food e.g. baking bread at 180oC for
minutes.
Remote controls
for TV, DVD players, garage door and curtain control in a house!
Infrared signal devices are used
as remote controllers for many household appliances and in industry too.
The
instructions are encoded in the infrared beam. Such devices work by sending out
a different signal pattern for each particular command eg for a TV and
recorder, each channel, stop, pause, play etc. will have their own unique
code transmitted in the infrared signal.
In a similar manner, infrared
beams can be used to transfer files between mobile phones or
laptops. However, the distance between the devices must be short and
the receiver must be in the direct line of sight of the transmitter.
You can design infrared security
systems
(i) You can set up an infrared
emitter and detector system that triggers an alarm if the signal is
interrupted (blocked) by an unwanted intruder on a property.
(ii) Thermal imaging security cameras
work well at night, when normal visible light cameras give poor imaging
e.g. if there was a night-time break-in at your home, thermal imaging
could provide accurate important video evidence of the intruders for
both the police and your home insurance provider.
Infrared can be used to transfer
information e.g. multiple telephone calls or TV signals through optical fibres at nearly the speed of
light!
Optic fibres are thin glass or
plastic strands that you can send a signal through carrying information
e.g. from computers, telephones and other data transfer systems.
The IR waves just bounces off the side of the thin strands of the
glass fibres (known as 'total internal reflection') and travels unimpeded
down the optical fibre with little loss due to absorption or scattering
of the wave energy on the side of the fibre optic cable.
The IR signal is transmitted into the optical
fibres, travels to the ends of them, and the signal picked up by a receiver.
Optical fibres can transfer information over very long distances.
Optical fibres often use a single
visible light wavelength/frequency carrier wave to reduce loss of information.
The digital information
signal is imposed on this infrared radiation carrier wave.
Cable television is delivered in this way.
Infrared cameras detect IR radiation and
build up a 'temperature picture' of what's in focus a bit like a visible
light camera does. The technique is called
thermal imaging.
The infrared radiation is converted into an electrical signal and
displayed on a screen.
You still see the shapes of objects but they are all
contoured in different colours depending on the temperature of the surface.
The hotter the object's surface is the brighter it appears on the
screen - you get contours of bands of different surface temperatures.
You photograph a house with an
infrared camera and detect where most heat loss is occurring.
Firefighters can use thermal
imaging cameras to look for the infrared emitted by warm bodies of
unconscious people in smoke-filled buildings - visible light is absorbed
or scattered by smoke particles, infrared is more penetrating.
Unlike visible light cameras, IR cameras
work off 'invisible' infrared radiation, and can be used in night-vision
cameras - security, nocturnal wildlife photography.
An increasingly important use is for
infrared cameras on low level orbiting satellites to monitor the use of
land e.g. crops, deforestation and the growth of urban areas -
particularly fast growing cities.
Different surface radiate or
absorb different amounts of infrared producing contours of slightly
different temperatures.
Heat sensors can detect infrared radiation -
safety device warning of overheating.
A greenhouse traps infrared radiation.
The higher frequency (shorter wavelength) infrared from the Sun passes
through the glass of the greenhouse warming the contents. The contents
re-radiate infrared radiation of lower frequency (longer wavelength) that
does not pass through the glass as easily, so more of the heat is trapped in
the greenhouse.
Narrow high intensity beams of infrared
can be used to cut through sheets of metal.
Infrared radiation can be used in
medical imaging, rather like the night vision camera, they detect areas
on the body that have increased in temperature due to an infection - again,
your are dealing with different intensities of infrared radiation producing
contours of slightly different temperatures. The technique also has the
advantage of monitoring the temperature over a wide region of the body very
quickly - no need for a thermometer.
Unfortunately, despite being safe to
use, infrared radiation is of very limited use in diagnostic medical
imaging.
Another 'domestic' case of infrared radiation! Unlike 'modern'
LED bulbs, 'old fashioned' filament bulbs emit quite a bit of IR heat
radiation. You can detect this with a frosty car where the central portion
of the ice has melted on the transparent headlamp cover. Filament bulbs only
convert ~10% of the electrical energy into visible light energy, most of the
rest is converted into infrared radiation. The ice absorbs infrared
equivalent to the latent heat of fusion (melting) and changes to liquid
water. Dangers of infrared radiation
Most infrared radiation is reflected or
absorbed by the skin. Infrared radiation is readily absorbed by
your skin and at high intensity will cause burns - from over exposure to
sunlight or too close to a radiant fire.
Infrared radiation contributes to
'heatstroke'/'sunstroke' when the body temperature rises over 40oC
(104oF).
This can occur in hot ambient
conditions, particularly if you are in bright sunshine and dehydrated.
Your body temperature is usually close to 37oC (98.6oF),
but in extreme conditions your
thermoregulation system
can fail. Initially you feel unwell (because you are!) and other
symptoms are confusion, red skin, headache and dizziness.
For humans and other warm-blooded
animals, excessive body temperature can disrupt enzymes regulating
biochemical reactions that are essential for cellular respiration and
the functioning of major organs
See also
Introduction to heat transfer - including infrared radiation
and
Absorption
& emission of radiation by
materials - temperature & surface factors including infrared
index:
radio waves *
microwaves
*
infrared radiation *
visible light
* ultraviolet
light *
X-rays *
gamma radiation
TOP OF PAGE and
full sub-index
(f)
The properties, uses
and dangers
of
visible light
– photography, animal vision
See also
visible light and colour
and
the structure and function of the eye
The sources and properties of visible
light
I think that the sources are pretty obvious
e.g. natural sunlight, artificial sources like lamps etc.
In these cases you are dealing with excited
energised atoms where electrons fall from a higher to a lower electronic
energy level (shell) the loss of energy given out as visible light EM
radiation.
White light consists of a mixture of all the
colours of the visible spectrum.
Know the colours of the he visible spectrum - red, orange, yellow,
green, blue, indigo, violet
and the trend ==> increasing frequency, decreasing wavelength,
increasing photon energy
The colour of an object depends on
which colours (wavelengths) are absorbed by, transmitted through or
reflected off, the surface of an object. For more details on colour see
...
The visible spectrum of colour, light filters and
explaining the colour of objects gcse physics revision notes
The uses of visible light
 Vision
- our eyes are very sensitive to light enabling our brain to construct an
image of what we are looking at - we take this for granted!
We use artificial light sources to illuminate the surroundings and objects
that we wouldn't see in the dark.
Photography: A camera basically consists of
an aperture (opens/closes) to let a controlled amount of light in and a lens
to collect and focus the light onto a light-sensitive film ('old way') or
electronic photocell screen ('new' digital way).
The amount of light
entering the camera is controlled by the shutter speed (time of exposure to
light) and the width of the aperture (f setting).
The photochemical process of light
sensitive silver compound film has mainly been replaced by light
sensitive screens in digital cameras.
In a digital camera, the signal from
the sensitive photocell is converted into an image that can be
transferred to e.g. a computer for storage, display, transfer to other
devices, further manipulation in software programs and printing out.
An indirect use! Plants use the visible
light of sunlight in photosynthesis and we rely on the plants for food!
BUT, not so indirect if you
deliberately use artificial light in greenhouses growing food on an
industrial scale - the plants can be grown continuously '24/7' to increase
efficiency.
Using
solar cells, visible (and uv) light can be directly converted into
electrical energy with a solar cell - so light is a renewable energy source.
Visible light can also travel down
optical fibres, the effect is used in some 'arty' decorative table lamps as
well as data transfer and communication.
The thin fibre can be made of
glass or plastic and is very long and flexible.
Optic fibre cables work because the
light signal is reflected off the internal surface with very little
absorption or scattering - the light rays literally bounce of the sides
of the fibre even if the fibre is bent - there is little loss due to absorption or scattering
of the wave energy on the side of the fibre optic cable.
The light waves in the fibre are
totally internally reflected off the internal surface and travel along
at the speed of light without being absorbed - the photons just bounce
of the internal surface of the fibre.
Optical fibres are used
for telephone communications and internet cables because you can encode
data onto the beam of light. Optical fibres can transfer
information over very long distances.
Optical fibres often use a single
infrared wavelength/frequency of the carrier wave to reduce loss of information.
The digital information signal is
imposed on this visible light carrier wave.
Optic fibres are also used in
medicine for internal examinations without intrusive surgery - if
necessary, only a small hole is needed. You send light down one set of
fibres to illuminate the tissue and a small camera records the images
and sends the picture back to the observer/computer screen. This helps
diagnose medical conditions. You can of
course also perform microsurgery with an attached small instrument.
The dangers of visible light
Most visible radiation is reflected or
absorbed by the skin.
Very intense visible light can damage
your eyes and people with very 'light sensitive' eyes wear shaded glasses to
reduce the intensity of visible (or uv, see next) light hitting the retina at the back
of the eye.
index:
radio waves *
microwaves
*
infrared radiation *
visible light
* ultraviolet
light *
X-rays *
gamma radiation
TOP OF PAGE and
full sub-index
(g) The properties, uses
and dangers of ultraviolet radiation
The sources and properties of ultraviolet radiation
Ultraviolet light is produced when a gas
is subjected to a high voltage discharge.
The atoms of the gas are excited
to a high electronic energy state - electrons are promoted to a higher
energy level (shell).
The electrons of the excited atoms drop down to lower
more stable electronic energy levels by losing energy in the form
ultraviolet FM radiation.
When ultraviolet light is absorbed, some
of the wave energy is converted into heat, BUT uv radiation can cause
ionisation - the process of knocking off outer electrons of atoms to
create positive ions (see
Appendix 1 for more details) - so uv light
is an ionising radiation.
Ultraviolet light can electronically excite
atoms or molecules so that they give off visible light photons as the electrons fall back down to
lower more stable energy levels - this is called fluorescence (an example of
luminescence).
The re-emitted radiation is of longer wavelength and lower
frequency of the EM radiation absorbed.
The absorption of uv light and
emission of visible light by molecules causes very bright 'fluorescent'
light colours to appear - the molecules are made to fluoresce by the uv
light.
Ultraviolet light is emitted by very hot
objects with temperatures of over 4000oC e.g. the Sun, but this
is not usually considered a practical source of uv radiation.
The uses of ultraviolet radiation
Small doses of ultraviolet rays are
good for us - they are absorbed by the skin and the energy helps in the
synthesis of vitamin D.
Vitamin D helps regulate the amount of calcium and
phosphate in the body. These nutrients are needed to keep bones, teeth
and muscles healthy. A lack of vitamin D can lead to bone deformities
such as rickets in children, and bone pain caused by a condition called
osteomalacia in adults.
Producing decorative bright fluorescent
colours with fluorescent materials (mechanism explained above).
Fluorescent lights use uv radiation to
make materials emit light - they are much more energy efficient than
filament bulbs for large scale multi-hour lighting e.g. in an office or
classroom.
The uv radiation is created by a high voltage discharge in a low
pressure gas, the excited electrons lose energy in the form of uv radiation.
The uv radiation strikes a fluorescent phosphorus coating on the inside of the glass
light tube where it is absorbed and re-emitted as visible light.
These kind of lights are energy
efficient and useful when lights are used for long periods of time e.g.
shops, factories and classrooms!
Very little, if any, uv light is
emitted from the outer surface of fluorescent lights.
Fluorescent materials
(often organic molecules), absorb higher energy ultraviolet radiation and
become 'excited'.
On relaxing to their normal lowest
energy state, the molecules re-emit radiation as visible light,
and that's what we call fluorescence.
You can mark objects with a
security pen with ink that is invisible in visible light.
When uv light is
shone on the ink markings they become visible due to the ink fluorescing.
This can be used to identify stolen property.
A similar technique is used to detect forgeries of bank notes and passports - the genuine bank
notes or passports are printed with special markings that only show up
when illuminated with uv light.
People give themselves an artificial sun-tan with UV lamps
in tanning salons or you can just sit out in the Sun which radiates
ultraviolet radiation.
These are a life-style choices
- definitely not any of mine - why take a risk?
Your skin naturally produces the dark pigment melanin,
and more so when exposed to extra uv light.
Melanin absorbs uv radiation to
protect skin cells from damage, but over exposure to uv can cause skin
damage - this happens particularly to pale coloured people who are exposed
to a lot of bright sunlight, hence a lot of uv radiation.
You must increase
your risk of skin damage, but it is a personal decision as to whether you
feel the risk of cancer is real enough for you to avoid the salon or sitting
out in the Sun without enough sun-blocker!
Since uv radiation can damage and kill cells, some
water treatment
plants sterilise the water by exposing it to uv radiation which kills harmful
bacteria.
The dangers of ultraviolet radiation
UV photons are of shorter
wavelength/higher frequency than visible light and so carry more energy and
are the first of three types of EM radiation in our sequence that can cause
ionisation and cause biological harm to cells - life!
UV photons have sufficient energy to
collide with molecules knocking off electrons - a process we call
ionisation.
Strong uv light can damage your eyes
and
possibly cause blindness - over exposure is not recommended!
People with very 'light sensitive' eyes wear
shaded glasses to reduce the intensity of uv (or visible) light hitting the
retina at the back of the eye.
Ultraviolet light can penetrate the skin
and be absorbed by the cells causing damage.
You can also suffer
from tissue damage (uv burn) or even radiation sickness.
This is why in bright sunlight you
should use sunscreens (sun-blockers), which absorb the harmful uv
radiation.
Ultraviolet light causes premature
aging of skin - wrinkles and darker pigmentation spots.
If the cell damage involves the DNA then
cancerous cells can multiply from the genetic mutation, which can lead to
skin cancer.
The melanin
in your skin is
an effective absorbent of uv light and this dark pigment can dissipate most
of the incoming uv radiation.
This is why fair-skinned people should be most
cautious out in bright sunlight and use sun-blocker appropriately.
You should know that the ozone layer
in the upper atmosphere partially protects us from potentially harmful
uv radiation.
UV radiation from the Sun is absorbed by
oxygen molecules (O2) to form ozone (O3) in the upper
atmosphere. Ozone molecules are very good absorbers of potentially damaging
uv radiation. Therefore the ozone layer of the Earth's atmosphere protects
us from the harmful effects of uv radiation - skin cell damage - burns and
genetic damage leading to skin cancer.
For more details see
Ozone, effect of CFC's, free radicals
notes.
See Appendix 1. section (d) for
general comments
on the dangers of ionising radiation
index:
radio waves *
microwaves
*
infrared radiation *
visible light
* ultraviolet
light *
X-rays *
gamma radiation
TOP OF PAGE and
full sub-index
(h)
The properties, uses
and dangers
of X-ray radiation
The sources and properties of X-ray radiation
X-rays are produced when a metal target is
bombarded with high energy electrons.
The target atoms are excited to a high
electronic energy state - electrons are promoted to a higher energy level
(shell).
The electrons of the excited atoms drop down to lower more stable
electronic energy levels by losing energy in the form X-ray FM radiation.
When EM X-ray radiation is absorbed, some
of the wave energy is converted into heat, BUT X-rays can cause ionisation - the process of knocking off outer electrons of atoms to
create positive ions (see
Appendix 1 for more details) - so X-rays
are an ionising radiation.
The uses of X-ray radiation
Medical uses of X-rays:
X-rays have a very short wavelength and
cause
ionisation ...
X-rays are absorbed by metal and
bone - but some X-rays pass through less dense material,
X-rays are mostly transmitted by less
dense healthy soft tissue, though there can be slight, but significantly
different absorptions between different tissues e.g. muscles and
organs.
X-rays affect a photographic film or
photoelectric screen (as in a digital camera)
in the same way as light, so an image can be built up based on absorption or
non-absorption of X-rays.
The wavelength of X-rays is of the same
order of magnitude as the diameter of an atom - means you can produce high
resolution images.
This means X-rays can be used to
diagnose and treat some medical conditions because they can be used to
examine the internal structure of the body.
CT scans: You use X-rays to produce
high
resolution 2D and 3D images of hard tissues in the body - you can detect
cancer tumours and bone fractures by this technique - X-ray photographs.
These images are called computerised tomography (CT) scans and are of a much higher
resolution than those using medical images from
ultrasound.
X-rays CT scans can detect bone fractures
detection, dental problems and cancer cell growths.
Using charge-coupled devices (CCDs) allows images to be formed electronically,
rather than with the 'old fashioned' photographic plate.
Be aware of precautions to be taken when
X-ray machines and CT scanners are in use, radiographers must be
particularly careful in their work.
More on X-raying your body to investigate bone
structure - we are definitely pentadactyl! When having an X-ray, the dose
should be as low as possible to minimise the risk of side-effects.
X-ray radiation is passed through the
object onto a detection screen and the image recorded. This was
originally a photographic plate, but now it is like a digital camera
screen producing a image file for storage and analysis.
The
more dense the bone, or
any other tissue, the more X-rays absorbed, hence the
differentiation in the image. Trained radiographers in hospitals will
take X-ray images to help doctors diagnose broken bones which show up
against lesser absorbing surrounding tissue. Any crack in the bone will
show up because more X-ray radiation will pass through the crack.
Note: You produce a negative
image where the brighter parts of the picture are where fewer X-rays
get through e.g. you see the dense bone clearly against the
background of the soft less dense, less X-ray absorbing tissue.
X-rays can also be used to
investigate internal organs e.g. to produce a mammogram when screening
for breast cancer. Here you are exposed to harmful radiation, but the
scan might save your life. Most people would accept a very low risk of
harm from X-rays compared to the risk of undiagnosed cancers.
In a similar fashion X-rays were used
in airport body/luggage scanners for security reasons.
However, these are banned
in some countries because of the potential harmful effects of X-rays.
Only a very low dose is used, but is the benefit of preventing a
terrorist incidence worth the risk of cancer?
You can now use
high-frequency radio wave scanners which are much safer.
The scientific technique of X-ray
crystallography is used to determine the internal structure of crystals to
see how the atoms, ions or molecules are arranged.
When X-rays pass through crystals
they create diffraction patterns that can be detected by photographic
techniques.
From the pattern you can work out
the position of the atoms in the crystal.
That's how we know the crystal
arrangement of ions in sodium chloride and how the double
helix structure of DNA was worked out.
The dangers of X-ray radiation
Doses of radiation
risk are measured in
sieverts
The quantity of radiation you are exposed to is
called the absorbed radiation dose and depends on where you live and
whether at work, you are likely to be exposed to harmful radiation
(e.g. radiographer, nuclear plant worker etc.).
The sievert dose unit (1 Sv =
1 J kg-1) is based on the
dose equivalent of ionising radiation. 1 sievert is quite a large dose of radiation, so
doses often quoted in mill-sieverts (1 Sv = 1000 mSv).
Radiation dose is not a measure of the total
amount of radiation your body absorbs, but it is a measure of the risk of
harm due to your body absorbing that amount of radiation. The risk depends on the
total amount of radiation
you absorb and how harmful that type of radiation is.
X-rays (and gamma rays) are the most
dangerous of the ionising radiations and easily cause tissue cell damage and
interfere with the function of cells e.g. can cause mutations - leading to cancer. X-rays have a very high energy and
are quite deeply penetrating in their energy transfer to the
absorbing material - which might be the deeper tissues and organs of the
body.
The risk of harm from an X-ray scan
is very low, but the risk of not diagnosing your injuries quickly and
accurately after an accident is much greater.
The risk (radiation dose) from a
CT scan
can vary from depending on which part of the body is scanned e.g. there
is a much greater risk from a chest scan compared to a head scan.
To minimise the chance of harm from
X-rays radiographers wear a lead apron and 'press the button' from behind a
protective lead screen. Without these precautions they would be
exposed to a large dose of radiation over time.
See Appendix 1. section (d) for
general comments
on the dangers of ionising radiation
index:
radio waves *
microwaves
*
infrared radiation *
visible light
* ultraviolet
light *
X-rays *
gamma radiation
TOP OF PAGE and
full sub-index
(i) The properties, uses
and dangers of gamma radiation
The sources and properties of gamma radiation
In the breakdown of the unstable nucleus in
radioactive decay, energy is released by the emission
(usually) of three types of ionising radiation (nuclear radiation) called alpha
particle radiation, beta particle radiation and gamma
ionising radiation.
Gamma radiation emission often
accompanies alpha and beta particle emission - its a way that a newly formed
and temporarily unstable nucleus gets rid of its excess energy to become
more stable.
Therefore you need a suitable
radioisotope that gives out gamma radiation when the atoms decay.
When EM gamma radiation is absorbed, some
of the wave energy is converted into heat, BUT gamma rays can cause
ionisation - the process of knocking off outer electrons of atoms to
create positive ions (see
Appendix 1 for more details) - so gamma
rays are an ionising radiation.
For more details on gamma radiation see:
What
is Radioactivity? Why does it happen? Three types of atomic-nuclear-ionising radiation
Alpha, beta & gamma radiation - properties of radioactive
nuclear emissions & symbols
The uses of gamma radiation
Gamma radiation can kill cells, but its
not all bad news - sterilisation.
Radioactive gamma ray sources are used to
sterilise
medical equipment like surgical instruments at room temperature. If a
microbe absorbs the gamma rays it is destroyed even if it is in a
microscopic crevice - gamma rays are very penetrating! This is much more
efficient that the old fashioned method of sterilising equipment in boiling
hot water - the heat might damage delicate equipment.
Packaged food can be sterilised in the
same way.
Any remaining microbes can be killed after the cooking and
packaging processes and once sterilised and sealed no microbes can get in to
rot and degrade the food.
This ensures the food is fresh for longer (longer shelf-life) and safe to
eat without having to preserve it any other way e.g. cooking or freezing.
Medical uses
As with sterilisation, gamma rays can be
used to kill harmful cells such as cancer cells.
A beam of gamma radiation
is directed through the body onto the cancer cells to kill them. The dose
must be the minimum required because its quite difficult to avoid killing
some healthy cells too. With most cancer treatments using gamma radiation,
your immune system takes a bit of battering and with some radiotherapy
treatments you can lose your hair.
As with X-rays, most people would
accept a risk of harm from gamma rays compared to the risk of leaving an
untreated cancer. Unfortunately, unlike having an X-ray where the
side-effects are negligible, the side-effects of gamma radiation
radiotherapy are quite substantial, but in most cases temporarily. This
is a classic case of risk versus benefit.
Since gamma radiation can damage and kill cells, some
water treatment
plants sterilise the water by exposing it to gamma radiation to kill harmful
bacteria.
Gamma rays are so powerful and
penetrating that they are transmitted through skin, soft tissue and even bone.
So gamma rays can be used in
medical imaging techniques.
If you can introduce a radioactive tracer into the body by
swallowing or injection, you can then monitor the movement of it.
Therefore gamma radiation is used in medical
imaging to help doctors diagnose certain kinds of health issues.
The person
is injected with a gamma emitter (radiotracer) which is so penetrating (unlike
alpha and beta radiation) that it comes out of the body and monitored on a
computer screen from the signals recorded by a gamma camera - a sort of
digital camera which is outside of the body. Y
You can then follow where
the tracer goes.
From the emitted gamma rays you can check on, for example, how efficient your blood
circulation is, your lung efficiency, but you need to be injected with a
gamma ray emitting radioisotope.
The radioactive tracer atom can be
part of a molecule normally present in the body like urea or glucose.
From where the tracer ends up doctors can see how efficient, or
otherwise, how the organ in the body is working.
e.g. cancer growths use more glucose
in respiration, they use more energy, therefore you see a 'hot spot'
where more radioactive glucose has accumulated.
Positron emission tomography (PET
scans) are used in medicine to produce highly detailed three-dimensional
images of the inside of the human body.
for more details
on PET scans and other uses of gamma radiation
... see
Uses of radioactive isotopes including gamma radiation in industry and
medicine notes
The
dangers of gamma radiation - the highest frequency and highest energy of
the EM spectrum
Gamma radiation (and X-rays) is the most
dangerous of the ionising radiations and easily causing tissue damage. DNA
can be damaged-altered interfering with cell function and cause mutations - leading to cancer.
Gamma rays have the highest EM
radiation energy and are very deeply penetrating in their energy
transfer to the absorbing material - which might be the deeper tissues and
organs of the body.
To minimise the chance of harm from
gamma-rays nurses and clinicians must take precautions e.g. protective
clothing, operating gamma ray machines by remote control - all to prevent
exposure to a large dose of radiation over time.
A high dose of gamma radiation kills many
cells quickly causing 'radiation sickness' - a serious general malfunction
of body leading to vomiting and hair loss, and can lead to death.
See Appendix 1. section (d) for
general comments
on the dangers of ionising radiation
Dangers of radioactive emissions -
health and safety issues and ionising radiation
index:
radio waves *
microwaves
*
infrared radiation *
visible light
* ultraviolet
light *
X-rays *
gamma radiation
TOP OF PAGE and
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(j) Appendix 1. Ionisation:
excitation of atoms and more on the dangers of ionising radiation
-
ionising radiation and dangers, electronic changes in atoms, emission of visible
light photons
KEY: EM shorthand for electromagnetic
(radiation); shell = electronic energy level; outer
means furthest from the nucleus; excited means an atom in a more
energised unstable electronic state. A photon is a little packet of EM
wave radiation (see
quantum theory for beginners on EM wave interactions!).
(i)
Reminders on the electronic structure
of atoms and how electrons can absorb energy
The electrons of atoms and molecules occupy a
series of specific electronic energy levels (shells) at increasing distance from
the nucleus. The electrons fill the lowest available energy levels nearest the
nucleus - the most stable arrangement. The electron arrangement of potassium is
shown on the right where the electrons fill three inner levels and two in
the outer shell (see
atomic structure). It is the outer shell electrons that are
the most easily lost in chemical reactions (see
Alkali Metals) or here due to ionising radiation (below).
The EM uv, X-ray and gamma radiations
have enough energy to promote outer shell electrons to a higher level
forming an electronically 'excited' atom or molecule. This can only happen
if the EM radiation has the specific amount of energy - if it isn't the
right energy, the electron can't absorb the energy and move to a higher
level (section (b) below). Sometimes the energy of the EM radiation is
sufficient to move an electron all the way up the energy levels and
completely remove it from the atom or molecule creating a positive atom or
molecule (section (c) below).
(ii)
Excitation of atoms by absorbing uv,
X-rays or gamma EM
radiation which then give out (emit) EM radiation
When the EM radiation has less energy
than that required for ionisation, you still 'excite' an atom into a more
energised state by promoting an electron from the outer shell to another, but
higher level shell (which may or may not be empty). The excited atom is unstable
and will 'relax' back to its normal stable state by emitting EM radiation
photons.
The diagram above illustrates the process:
1. An
electron in an outer shell absorbs incoming EM radiation energy and is
promoted up to the next higher empty shell (in this case, but can be a partially
filled shell). This called electron 'excitation'.
2. The
atom is now in an 'excited' state and unstable because the
electron has gained excess energy and if possible the atom would like to return
its original stable state.
3.
So, the
promoted electron now loses energy and drops back down to the 'stable'
original level stabilising the atom.
The excess energy is lost as EM radiation
and the photon emitted has the same
energy as that absorbed by the electron in the first place.
See section on
flame emission spectroscopy and
flame colours notes and make the connection!
and
Atomic structure, history, definitions,
examples and explanations including isotopes
In general, when the electrons in
electronically excited atoms or molecules fall from a higher shell to a lower
shell EM radiation is emitted in the form of visible light, ultraviolet light or
X-rays depending on the energy difference of the levels.
The higher the level (shell) the electron
is promoted too, the greater the energy and frequency of the EM radiation
emitted when the electron falls down to lowest possible level.
However, these processes are complicated
with many different electron transitions possible.
The further you are from the nucleus,
the closer the energy levels become e.g. an electron falling from the
5th level to the 4th level releases a more energetic higher frequency
photon, than an electron falling from the 6th level to the 5th etc.
If you can raise an electron to
the highest possible level (which amounts to 'infinity' since there
an infinite number of levels theoretically), it has sufficient energy to
overcome the attraction of the nucleus. In other words the atom loses
an electron and forms a positive ion - because with one negative
electron charge less, there is now a surplus of positive proton charge
in nucleus. This process is described further with diagrams in the next
section (c).
NOTE danger! The excited atoms that
can be dangerous and promote chemical reactions you might not wish to happen
(e.g. in living cells) and also release their excess energy as heat.
(iii)
Ionisation by EM radiation of atoms to
form a positive ion
The higher energy uv, and both X-ray
and gamma radiations have enough energy to cause complete ionisation.
An atom is ionised if it completely loses
one or more electrons.
This is a bit more energetic than
'excitation' described above.
The energy carried by ultraviolet light
radiation, X-rays and gamma radiation is sufficient to cause ionisation of atoms
by knocking off negative outer shell electrons to form a positive ion - the atom
has been ionised - see diagrams and explanation below.
== high energy uv/X-ray/gamma ray photon ==>
+
+ electron-
This represents the ionisation of a sodium
atom to form a positive sodium ion and a free electron:
Na ==> Na+ + eŻ
(electron configuration change of sodium from 2.8.1 ==> 2.8, as in chemistry
notes!)
In this case the incoming EM radiation must
have sufficient energy to promote the electron all the way up the energy levels
until it is completely free of the attraction of the positive nucleus - so the
atom has been ionised.
A positive ion is formed because there are
now less negative electrons on the atom than positive protons - so there is a
surplus of positive charge on the atom. The charge on the ion can be +, 2+, 3+
etc. by knocking off 1, 2 or 3 electrons etc. The more electrons knocked off,
the bigger the positive charge on the ion.
(Note: Using X-rays, you can knock off all 92
electrons from a uranium atom, element 92, 92U, to form the U92+
ion, but this far too extreme for GCSE students, but very exciting to
contemplate!)
Dangers of ionising radiation
From (b) we see that excited atoms that can
be dangerous and promote chemical reactions you might not wish to happen.
BUT, ions can be even more destructive on
cells and break chemical bonds and cause even more genetic damage.
This is the essence of the dangers of
ionising radiation - burns and cell DNA damage (mutations) leading to cell death or rogue
multiplication of mutated cancer cells.
(iv) Although NOT needed for GCSE science
level - a note on 'Quantum Theory' hmm!!!
You can skip to (v) dangers
of ionising radiation!
A photon is a tiny packet
of EM radiation energy, it has both particle and wave properties. You can think
of it as a little bullet of energy in wave-like form (~).
The diagram above in (b) illustrates an
example of the scientific theory we call quantum physics. It
describes the interaction and exchange of energy between the electrons of
atoms and molecules with photons of EM radiation. The diagram actually shows
one atom of sodium interacting with one photon of EM radiation.
Obviously overall you are dealing with trillions of atoms and photons of EM
radiation, BUT, it occurs at an individual atom level - at the quantum
level - so, what's a quantum?
A photon is sometimes called a 'quantum' of energy -
this term and 'quanta' (plural of quantum) are derived from the early theoretical ideas of quantum
physics which suggested, (correctly), that energy changes involve the exchange
of tiny packets of energy called 'quanta'.
e.g. as you are reading this web
page, trillions of visible light photons (quanta of EM radiation energy of
wavelength 400-700 nm) from the screen are hitting the retina at the back of
your eye to excite the molecules in the receptor cells! Here the 'excitation'
effect doesn't lead to emitted EM radiation, but does create a nerve signal
to the brain - a tiny quantum energy transfer per photon.
We are all quantised at
the atomic and molecular level - scary!!!
So, reading section (b) for your GCSE
exam, is probably your first encounter with (perhaps the last!), of what
some scientists regard as the most successful theory of all science of all
time - quantum theory!
(v)
More on the dangers of
ionising radiations
High energy ultraviolet light, and even
higher energy X-rays and gamma radiation are all types of ionising radiation -
these EM waves carry enough energy to remove electrons from atoms and molecules.
These ionised atoms and molecules are very
reactive and can cause all sorts of reactions to happen in cells that would not
have otherwise occurred.
These reactions may be harmful to the life of
a cell - it can be damaged or killed.
These reactions can cause mutations in the
cell DNA that can lead to cancer.
If the ionising radiation kills cells,
but not too many, you can survive without any long-term effect.
A very high dose of e.g. gamma radiation,
can kill so many cells and damage others that your immune system is
overwhelmed and you suffer from radiation sickness and your life is in
peril.
BUT, if the radiation just damages a cell
and causes a DNA mutation, this can be carried forward by uncontrolled
cell division and if the damaged cells are cancerous, then a tumour can
grow in your body with potentially fatal consequences
Exposure to high levels of ionising
radiation can be quite dangerous to us and other animals and pants.
For more specific details on dangers see
ultraviolet
light *
X-rays *
gamma radiation
index:
radio waves *
microwaves
*
infrared radiation *
visible light
* ultraviolet
light *
X-rays *
gamma radiation
TOP OF PAGE and
full sub-index
Check out your
practical work you did or teacher demonstrations you observed, all of this is part of good revision for your
module examination context questions and helps with 'how science works'.
investigating the range of
Bluetooth or infrared communications between mobile phones and laptops,
(k) Some general learning objectives
-
Know and understand that electromagnetic radiations
travel as waves and move energy from one place to another.
-
Know that these radiations can all
travel through a vacuum and do so at the same speed.
-
Know and understand that waves cover a continuous
range of wavelengths called the electromagnetic spectrum.
-
Know
that current evidence suggests that the
universe
is expanding (separate page) and that matter and space expanded
violently and rapidly from a very small initial ‘point’, ie the universe
began with a ‘big bang’.
-
You are expected to use your
skills, knowledge and understanding to:
-
compare the uses and properties of different types
of waves for communication,
-
your expected knowledge and understanding of
waves used for communication is limited to
sound
(separate page), light, microwaves, radio
waves and infrared waves from the electromagnetic spectrum.
-
evaluate the possible risks involving the use
of mobile phones,
-
and consider the limitations of the model that scientists use
to explain how the universe began and why the universe continues to expand.
-
Be able to understand that the potential
danger associated with an electromagnetic wave increases with increasing
frequency.
-
Be able to relate the harmful effects, to
life, of excessive exposure to the frequency of the electromagnetic
radiation, including:
-
a) microwaves: internal heating of body
cells
-
b) infrared: skin burns
-
c) ultraviolet: damage to surface cells and
eyes, leading to skin cancer and eye conditions
-
d) X-rays and gamma rays: mutation or damage
to cells in the body
-
Be able to describe some uses of
electromagnetic radiation:
-
a) radio waves: including broadcasting,
communications and satellite transmissions
-
b) microwaves: including cooking,
communications and satellite transmissions
-
c) infrared: including cooking, thermal
imaging, short range communications, optical fibres, television remote
controls and security systems
-
d) visible light: including vision,
photography, uses of optical lenses and illumination
-
e) ultraviolet: including security marking,
fluorescent lamps, detecting forged bank notes and disinfecting water
-
f) X-rays: including observing the internal
structure of objects, airport security scanners and medical X-rays
-
g) gamma rays: including sterilising food
and medical equipment, and the detection of cancer and its treatment
-
Know that ionising radiations are
emitted all the time by radioactive sources
-
You should know that ionising radiation
includes alpha and beta particles and gamma rays and that they transfer
energy.
-
Know that radio waves, microwaves, infrared and visible light
can be used for communication.
-
You will be expected to be familiar with
situations in which such waves are typically used and any associated
hazards.
Some learning objectives for medical physics
-
Appreciate that physics has many applications in
the field of medicine.
-
Know that these include the uses of X-rays
and ultrasound for scanning, and of light for image formation with lenses
and endoscopes
-
evaluate the use of different
lenses for the correction of defects of vision,
-
compare the medical use of
ultrasound and X rays,
-
evaluate the advantages and disadvantages of using
ultrasound, X-rays and Computerised Tomography (CT) scans,
TOP OF PAGE and
full sub-index
WAVES - electromagnetic radiation, sound, optics-lenses, light and astronomy revision notes index
General
introduction to the types and properties of waves, ripple tank expts, how to do
wave calculations
Illuminated & self-luminous objects, reflection visible light,
ray box experiments, ray diagrams, mirror uses
Refraction and diffraction, the visible light
spectrum, prism investigations, ray diagrams explained
gcse physics
Electromagnetic spectrum,
sources, types, properties, uses (including medical) and dangers gcse physics
The absorption and emission of radiation by
materials - temperature & surface factors including global warming
See also
Global warming, climate change,
reducing our carbon footprint from fossil fuel burning gcse
chemistry
Optics - types of lenses (convex, concave, uses),
experiments and ray
diagrams, correction of eye defects
The visible spectrum of colour, light filters and
explaining the colour of objects gcse physics revision notes
Sound waves, properties explained, speed measure,
uses of sound, ultrasound, infrasound, earthquake waves
The Structure of the Earth, crust, mantle, core and earthquake waves (seismic wave
analysis)
gcse notes
Astronomy - solar system, stars, galaxies and
use of telescopes and satellites gcse physics revision notes
The life cycle of stars - mainly worked out from emitted
electromagnetic radiation gcse physics revision notes
Cosmology - the
Big Bang Theory of the Universe, the red-shift & microwave background radiation gcse
physics
IGCSE revision
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