Units

Wavelength

The electromagnetic spectrum covers a very wide range of wavelengths. Some, like long-wave radio waves, are over a kilometre. Others, like Gamma rays, have extremely small wavelengths, to measure these we need tiny units which you may not have met before.

Here’s a summary of units used for wavelength -

kilometres	km	1km = 1000 metres.
metres	m	You’re probably between 1½ and 2 metres tall
centimetres	cm	100cm = 1m
millimetres	mm	1,000mm =1m
nanometres	nm	1nm = 1 thousand millionth of a metre -a millionth of a mm.

So 1,000nm = 0.001mm

Frequency

A wide range of wavelengths means a wide range of frequencies.

 Here’s a summary of units used for frequency -

Gigahertz	GHz	1GHz = 1 thousand million waves per second.
Megahertz	MHz	1MHz = 1 million waves per second
Kilohertz	kHz	1kHz = 1 thousand waves per second
Hertz	Hz	1Hz = 1 wave per second

Dealing with large and small numbers

You may come across wavelengths written as "2 x 10 to the power of -9 m" or frequencies as "1 x 10 to the power of 10 Hz" - this is called Scientific Notation, and is a short way to express very large and very small numbers.

Basically the number "1x10 to the power of 6" means "1 followed by 6 zeros" - in other words 1,000,000 .
So "1 x 10 to the power of 3" is 1,000  (3 zeros)
and "1 x 10 to the power of 9" is 1,000,000,000.

The number "1 x 10 to the power of -1" means "1 divided by 10" - in other words 0.1
So "1 x 10 to the power of -2"  is   1 divided by 100 = 0.01
Thus 1 nm can be written as "1 x 10 to the power of -9m".

At first this may seem like a complicated way to write a simple thing, but it means you don’t spend ages writing out great long strings of zeros - and also you’re less likely to make mistakes by mis-counting the zeros. It also enable you to use short cuts and make estimates more easily.


wavelength+frequency
Wavelength and Frequency

Think of waves on the sea. we call the distance from the top of one wave to the top of the next one the wavelength. This is the same as the distance between the bottoms of the waves.
We measure it in metres.

The Frequency of a wave is simply "how many waves go past in 1 second". We measure it in Hertz (Hz) - 1Hz is 1 wave per second.

Jump to Explanation of Units

Wavelength and frequency are linked - a wave with a long wavelength will have a low frequency, and a high frequency wave will have a short wavelength.

Equation


types-of-wave
Types of electromagnetic wave and their wavelengths

The electromagnetic spectrum covers a sliding scale of wavelengths, ranging from very short to very long.
We divide it into sections and give each section a name -

(Very short waves)                                          (Typical wavelength)
                               Gamma Rays                               0.01nm
                               X Rays                                             1nm
                               Ultra-Violet                                  100nm
                               Visible Light                        400 - 700nm
                               Infra-Red                                    0.01mm
                               Microwaves                                      1cm
                               UHF Radio                                         1m
                               VHF Radio                                     100m
                                Long & medium wave radio             1km
(Very long)

How am I supposed to remember this?

Jump to explanation of units

UHF stands for "Ultra High Frequency", and VHF stands for "Very High Frequency" - they're just high frequency radio waves.

The top of this table is the high frequency, short wavelength waves.

The bottom is the low frequency, long wavelength waves.


contents
Broadoak School, Weston-super-Mare
GCSE (SEG) Physics.

In your exams, you will be expected to know about :

1 - The NAMES of the different types of wave
that make up the electromagnetic spectrum.
2 - The ORDER in which they fit into the spectrum.
3 - What they can be USED for.
4 - How they are MADE.
5 - The DANGERS of each.

All the information you need is in this file.
You could make notes on it, or do it as a table.

At the top of the screen, the "Contents", "Search" & "Back" buttons are handy.

Clicking on anything GREEN will make things happen.

Use the scrollbar to see the rest ------->>>

    The electromagnetic spectrum is made up of what appear to be several different types of wave, but in fact the only difference between them is their frequency.
    In other words, visible light, Gamma rays and your local radio station broadcasts are all made of the same stuff - just higher or lower frequencies.
    You could think of a very long piano keyboard - then Gamma rays are the high notes & radio waves are the low notes. Visible light is somewhere in the middle.

Contents

Click on a heading to jump to it

> The types of wave, and their wavelengths

> Uses of electromagnetic waves

> Where do the waves come from?

> Dangers

> Explanation of units

> Explanation of wavelength and frequency

> Quick revision quiz

When you've finished, click on "File", then "Exit".


Like this!


uses
Uses of Electromagnetic Waves

Long wave and Medium wave radio waves are used for communications.

VHF radio is around 100MHz - used by FM radio stations. It allows better quality transmissions than long or medium wave.

UHF radio waves are used by TV transmitters and Police radios. It’s a shorter wavelength (higher frequency) than VHF.

Microwaves can be used to heat food. It so happens that fat, water and sugar molecules can readily take in energy from microwaves, causing the molecules to vibrate. If molecules are vibrating more, then the substance is hotter. Dishes tend not to get hot in a microwave oven because they don’t contain those sorts of molecule. But YOU contain a lot of those molecules, so microwaves will heat you up!
Also microwaves are used for radar, and for communications links between Telecom towers because they can be sent in a tight beam. Television Outside Broadcasts often use microwave dish aerials to get live pictures to the nearest Telecom tower and thus back to base.

Infra red radiation is heat radiation. So it can be used to heat things up - some people find that an infra red lamp relieves muscular aches & pains. It’s also easily produced by LEDs (Light Emitting Diodes), so we use it to send signals from your TV remote control to the TV. As it’s given off by warm objects such as people, we can use infra red sensors to trigger alarms, or infra red cameras to see intruders or wildlife at night, or for emergency services to spot survivors under collapsed buildings and to see through smoke. (These are called "Thermal Imaging Cameras", and one was called in to help us trace a leak from heating pipes under Mrs Joyce's office in spring 1996).

Visible light is useful for seeing where you’re going!

Some substances glow ("fluoresce") when Ultra-violet light strikes them - certain inks do this, so an ultra-violet lamp can detect forged banknotes. They can also give you a sun tan - but you ought to read the section on "dangers" about this!

Jump to "Dangers" section

X rays can pass through many substances, but some substances are easier than others. They
also make photographic film turn black. Thus if you are between a source of X rays and a piece
of film, the film will go black everywhere except for the places where your bones got in the way
- and the doctor can see any cracks or breaks. Higher-energy X rays can pass through metals,
and be used to inspect the insides of, for example, aircraft parts.

Gamma rays can pass through most substances, and because of this they can be used to detect cracks and other imperfections deep within objects. Also they kill bacteria and other living cells, so they can be used to sterilise medical equipment, make food keep longer, and to kill cancers. Unlike X rays they are produced naturally from pieces of some radioactive materials - you don’t need a complicated machine to make them.


Dangers

The short-wavelength (high frequency) waves are the dangerous ones, although high doses of any electromagnetic wave can be harmful - for example, public concern has recently grown over houses and schools near overhead power lines.

 Gamma rays damage the cells in your body and thus cause cancer - however they can be used in carefully controlled beams to kill only the cells they’re aimed at, and so kill cancerous cells and stop tumours spreading. Gamma rays can be stopped by thick concrete or lead shielding.

X rays can also be harmful, especially to unborn children, so doctors and dentists tend to avoid using them on pregnant women. Like Gamma rays, X rays interfere with the way that cells develop. Small doses are very unlikely to harm you, so it’s OK for medical use - but the people who operate the machines need to stand behind a shield to avoid building up a dangerous dose. Lead is good at stopping X rays, so the glass panel in their shield is made of glass that contains lead. They often wear lead aprons as well.

Over the past few years we have become more aware of the dangers of Ultra-violet rays. In small doses they cause the production of pigment (a brown colour, called melanin) in skin cells, which we call a sun tan. Larger doses cause sunburn, larger doses lead to skin cancer. Think of a sun tan as a radiation burn!

Too much visible light can damage your retinas.

Infra-red is heat radiation, large amounts make you hot! You can feel it coming from the Sun.

Microwaves can be used to heat food. It so happens that fat, water and sugar molecules can readily take in energy from microwaves, causing the molecules to vibrate. If molecules are vibrating more, then the substance is hotter. Dishes tend not to get hot in a microwave oven because they don’t contain those sorts of molecule. But YOU contain lots of them! As microwaves are also used for radar, people who work around aircraft may have to wear reflective clothing (for example on the deck of an aircraft carrier).


where-they're-made
Where do the waves come from?

Gamma rays are produced by some nuclear reactions, including those inside stars. They are very high-energy waves. They are also produced by the radioactive decay of some substances. If they've arrived here from space we call them cosmic rays.

X rays are also produced in stars, but we can make them by firing a beam of electrons at a metal target. The beam and the target have to be in a vacuum tube, or air will slow the electrons down too much. The harder we fire the electrons at the target, the higher-energy X rays we produce, and the better they are at penetrating materials.

Ultra violet rays come from extremely hot objects. As well as coming from the Sun, they can be made by certain types of fluorescent lamp.

Visible light is given off by stars and hot objects - such as the filament of a light bulb (about 3000 degrees Celsius).

Infra red radiation is produced by anything that is warmer than its surroundings. This includes central heating systems, Bunsen burners, and YOU (which is how security lights detect you). Also made by stars, such as the Sun.

Microwaves and the various radio waves are produced by radio transmitters.

Stars produce all of these waves in huge quantities.


Wavelength and frequency are linked by the equation -

speed = frequency x wavelength

This is true for ANY wave.


mnemonic
Don't panic - you're not expected to remember all the numbers.

But you are expected to remember the names & the order.
     Try this, or make up your own !

          Giraffes get                      ...........      Gamma rays
        eXcited by                          ...........       X rays
          Undressing                       ...........       Ultra violet
          Very                                  ...........      Visible light
          Impatient                          ...........       Infra red
          Martian                            ...........       Microwaves
          Rodents                            ...........       Radio


quiz
Revision quiz

1)  X rays have a higher frequency than Gamma rays. True or False?

True

False

2)  Microwaves have a shorter wavelength that radio waves. True or False?

True

False

3)  X rays are used for radar. True or False?

True

False

4)  Which one of these do we use for the remote control for your television set?

Microwaves

Ultra violet

Infra red

5)  Ultra violet waves are used in domestic microwave ovens. True or False?

True

False

6)  In general, the most dangerous waves are the high frequency ones. True or False?

True

False

Use the "Contents" or "Back" buttons at the top of the screen.


Correct


No - try again


No, Microwaves are used for radar.


No - why do you think we call them "Microwave" ovens ?