ELECTROMAGNETIC RADIATION - introduction to EM waves

types, properties, uses and the spectrum of visible light

Currently being enlarged & re-edited, will be ready by Fri 21st Oct 2018

Doc Brown's Physics Revision Notes

Suitable for GCSE/IGCSE Physics/Science courses or their equivalent

 This page will answer many questions about the electromagnetic spectrum.

 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 continuous electromagnetic spectrum including in order ...

radio waves  *  microwaves   *  infrared radiation  *  visible light  *  ultraviolet light  *  X-rays  *  gamma radiation

and the trend ==> increasing frequency, decreasing wavelength, increasing energy

(visible light includes the colours of the visible spectrum - red, orange, yellow, green, blue, indigo, violet)



Use of waves for communication and to provide evidence that the universe is expanding

  • 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 use 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.

    • 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 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.

 

All the properties and uses of electromagnetic radiations are described in detail in individual sections

radio waves  *  microwaves   *  infrared radiation  *  visible light  *  ultraviolet light  *  X-rays  *  gamma radiation

 

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Some more general points on electromagnetic radiation we are talking about ...

radio waves, microwaves, infrared radiation (ir), visible light spectrum, ultraviolet light (uv), X-rays and gamma radiation

Electromagnetic (EM) radiations are an example of transverse waves.

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.

However, in passing into transparent dense materials the speed is considerably 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 a medium until they are absorbed.

Therefore we are talking about energy store exchanges

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 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.

BUT the 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 radiation.

All EM radiations are oscillating electric and magnetic fields.

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 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 tried to mention 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 what is the substitute for the humble X-ray?

radio waves  *  microwaves   *  infrared radiation  *  visible light  *  ultraviolet light  *  X-rays  *  gamma radiation

Need for notes on how Herschel and Ritter contributed to the discovery of waves outside the limits of the visible spectrum 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 are all part of daily life!

 

A greater range of the electromagnetic spectrum - seven varieties

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, X-rays and gamma rays would not normally be part of daily life!

The energy of the electromagnetic photons is directly proportional to their frequency.


All the properties and uses of electromagnetic radiations are described in detail in individual sections

radio waves  *  microwaves   *  infrared radiation  *  visible light  *  ultraviolet light  *  X-rays  *  gamma radiation


Wave calculations

You should know that all waves obey the same wave equation:

v = f x λ  where

v is speed in metres per second, m/s, which for electromagnetic radiation in vacuum/air is ~3.0 x 108 m/s

f is frequency in hertz, Hz (per sec)

λ  is wavelength in metres, m

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 = 3 x 108 m/s

Be able to do examples of calculations using the wave speed formula and its rearrangements.

 

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The properties and uses of electromagnetic radiations (in order of increasing frequency)

radio waves  *  microwaves   *  infrared radiation  *  visible light  *  ultraviolet light  *  X-rays  *  gamma radiation

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 produce an oscillating electric and magnetic field that produces-emits an EM wave of radiation. 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 what we call radio waves.

Receiving radio waves: EM waves can cause charged particles like electrons to oscillate at the same frequency. If these electrons are part of an electrical circuit, an alternating current is induced at the same frequency as the EM wave - this is what a radio receiver aerial does and the rest of the electronics does the rest to produce the sound and pictures.

The combination of transmitter and receiver allows you to encode information onto a radio signal and transmit information from one place to another - its essentially a data transfer system.

 

Uses of radio waves

Radio waves are used to transmit from one location to another the information your TV and radio 'appliances' need for you to view and listen to.

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.

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 (== /\/\/\ ==>).

 

Dangers of radio waves

I don't know of any? Does political propaganda count?!!!

 


 

The properties and uses 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 fields (no details required at GCSE level).

 

Uses of microwave radiation

TV transmitters/receiver sets use microwave signals via satellite communication ('satellite TV') and mobile phones ('satellite phone') can use satellite communication systems too, as well as local transmitting/receiving 'mobile phone' masts that are now appearing everywhere!

The microwave signal must have a wavelength to allow it to penetrate clouds (water droplets) and water vapour to communicate with satellites in orbit thousands of km above the Earth's surface.

The microwave signal (telephone or TV) is transmitted through the Earth's atmosphere into space where the satellites receiver dish picks up the signal which is then re-transmitted back to a receiver on the Earth's surface eg TV satellite dish and detector.

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.

Microwave mobile phone call signals are picked by, or transmitted by, the nearest 'mobile phone mast' receiver/transmitter.

Microwave cookers use EM radiation to heat up food and pretty rapidly as the high frequency microwaves readily penetrates the food.

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!

 

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.

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!), and so potentially, since you contain a lot of water, 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.

 


 

The properties and uses of infrared waves (IR radiation) – remote controls for TV, DVD players, garage door and curtain control in a house!

The sources and properties of infrared radiation

To obtain a viable source of FM infrared radiation ('heat radiation') you need an energy store at a higher temperature than the background e.g. hot water radiator, electric heater etc. When you heat up materials the bonds between the atoms in the molecules vibrate more energetically, and so the molecules are more unstable with respect to the cooler less 'vibrating' background molecules. When the vibrations decrease, the energy is released by the material emitting FM infrared radiation.

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.

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.

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.

Infrared can be used to transfer multiple telephone calls through optical fibres at nearly the speed of light! The IR waves just bounces off the side of the thin strands of the glass fibres (known as 'total internal reflection') and so travels unimpeded down the optical fibre. The IR signal is transmitted into the optical fibres, travels to the ends of them, and the signal picked up by a receiver. 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 infrared 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.

Unlike visible light cameras, IR cameras work off 'invisible' infrared radiation, and can be used in night-vision cameras - security, nocturnal wildlife photography.

Heat sensors can infrared radiation - safety device warning of overheating.

Narrow high intensity beams of infrared can be used to cut through sheets of metal.

 

Dangers of infrared radiation

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.


 

The properties and uses of visible light – photography, animal vision

The sources and properties of visible light

I think that the sources are pretty obvious e.g. natural sunlight, artificial sources e.g. lamps etc.

In all cases you are dealing with excited energised atoms where electrons fall from a higher to a lower electronic energy level (shell).

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 uses of visible light

We use artificial light sources to illuminate the surroundings and objects that we won't see in the dark.

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 mostly been replaced by light sensitive screens in digital cameras.

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 to increase efficiency.

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 light waves in the fibre are totally internally reflected off the internal surface and travel along at the speed of light without being absorbed. Optical fibres are used for telephone communications and internet cables

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. You can of course also perform microsurgery with an attached small instrument.

 

The dangers of visible light

Strong 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) light hitting the retina at the back of the eye.

 

scan spectrum image


The properties and uses 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 so that they give off visible light as the electrons fall back down to lower 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 uses of ultraviolet radiation

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.

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.

People give themselves an artificial sun-tan with UV lamps in tanning salons. 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!

Since uv radiation can damage cells, some water treatment plants sterilise the water by exposing it to uv radiation to kill harmful bacteria.

 

The dangers of ultraviolet radiation

Strong uv light can damage your eyes and possibly cause blindness. 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. If the cell damage involves the DNA then cancerous cells can multiply. 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


The properties and uses of X-ray radiation

The sources and properties of X-ray radiation

X-rays are produced when a 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

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 and saved image file.

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 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 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?

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. That's how the double helix structure of DNA was worked out.

 

The dangers of X-ray radiation

X-rays (and gamma rays) are the most dangerous of the ionising radiations and easily damage the function of cells and 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.

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

 


The properties and uses 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 newly formed and temporarily unstable nucleus gets rid of its excess energy to become more stable.

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. 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. 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.

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.

Gamma radiation is used in medical imaging to help doctors diagnose certain kinds of health issues. The person is injected with a gamma emitter. Gamma radiation is so penetrating (unlike alpha and beta radiation) that it comes out of the body and monitored on a detection screen. You can check on, for example, how efficient your blood circulation is, your lung efficiency.

See Uses of radioactive isotopes including gamma radiation in industry and medicine notes

 

The dangers of gamma radiation

Gamma radiation (and X-rays) are the most dangerous of the ionising radiations and easily damage the function of cells 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.

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

 


Appendix 1. Ionisation - 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!).


(c) doc b(a) 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).


(b) Excitation of atoms by absorbing 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).

2. The atom is now in an 'excited' state and unstable because the electron has gained excess energy.

3. The promoted electron now loses energy and drops back down to the 'stable' original level. The excess energy is lost as EM radiation of 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!

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 bigger the difference in energy level the greater the frequency and energy of the emitted radiation and the potentially more harmful it is to human beings (see section (d) dangers).


Although NOT needed for GCSE science level - a note on 'Quantum Theory' hmm!!!  skip to (c) ionisation!

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!


(c) 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.

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.

  ==  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+  +  e¯  (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!)


(d) 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


radio waves  *  microwaves   *  infrared radiation  *  visible light  *  ultraviolet light  *  X-rays  *  gamma radiation


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,

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