Electromagnetic spectrum: 4.
The properties, uses
Doc Brown's Physics exam study revision notes
INDEX of physics notes: Properties
and uses of
The properties, uses
microwave radiation for mobile phones and satellite television
The sources and properties of
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
(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
Microwave mobile phone call
signals are usually picked by, or transmitted by, the nearest 'mobile phone mast'
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
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
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
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.
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.
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
(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
of electromagnetic radiation = 3.00 x 108 m/s.
(a) What is
the frequency of the microwave beam?
wavelength x frequency
f = v ÷ λ =
3.00 x 108 ÷ (1.20 / 100) =
2.50 x 1010
(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
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.
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 of notes: Properties and
Keywords, phrases and learning objectives for electromagnetic radiation spectrum
Be able to describe microwaves in terms of their properties, sources
as part of the electromagnetic spectrum.
Know and be able to describe and explain the uses of
microwave radiation including radar scanners, microwave
cookers, mobile phone and TV communications.
Know that an intense beam of microwaves can causes burns
- one of the few dangers of microwave radiation.
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INDEX of notes: Properties and