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GCSE Physics · GCSEChapter 19 · The electromagnetic spectrum
GCSE · Chapter 19

The electromagnetic spectrum

By the end of this chapter you will be able to

  • Put the seven types in order of wavelength and frequency, and say which has the most energy
  • Match a use to the type of wave that is used
  • Explain why gamma and X-rays are more dangerous than radio waves, and what protects against them
  • [H] Calculate the wavelength or frequency of an electromagnetic wave

You should already know

  • Transverse waves and wave speed (Chapter 17)
  • Standard form and powers of ten (KS3 Maths)

Where this leads

  • Reflection, refraction and lenses (Chapter 18, which uses light)
  • Radioactivity and ionising radiation (Chapter 11)

GCSE Physics. Content follows the GCSE (9-1) subject content for England (Edexcel and AQA). Content references: Order the seven types of electromagnetic wave by wavelength and frequency: radio, microwave, infrared, visible, ultraviolet, X-ray and gamma, State that all electromagnetic waves travel at the speed of light in a vacuum, about 3.0 x 10^8 m/s, Describe uses of each type of electromagnetic wave, Describe the dangers of ultraviolet, X-ray, gamma, microwave and infrared radiation, and how to reduce them, [H] Use v = f lambda with c = 3.0 x 10^8 m/s to calculate the wavelength or frequency of an electromagnetic wave, Explain the greenhouse effect as the absorption and re-emission of infrared radiation by gases (idea).

Exercise P  Do you remember?

1
FWhich type of wave is transverse: light or sound in air? Write the word.
2
PSWhat is 3 x 10 to the power 8 divided by 10 to the power 6? Write the number.
3
FWhat is the unit of wavelength? Write the unit.
4
FWhat is 2 x 10 to the power minus 9, written as a decimal? Write the number.
5
FName the energy store of the food in the body. Write the word.
19.1

The seven types of electromagnetic wave

GCSE
Order the seven types of electromagnetic wave by wavelength and frequency, State that all electromagnetic waves travel at the speed of light in a vacuum
Video lesson: The seven types of electromagnetic waveComing soon
Context. A radio plays music in the kitchen, a microwave heats the soup, and a torch shines light. “Are these all the same kind of wave?” asks Ahmed. Yes: they are all electromagnetic waves. They differ only in their wavelength and frequency.

Electromagnetic (EM) waves are transverse waves that do not need a medium, so they can travel through a vacuum. In a vacuum they all travel at the speed of light, about 3.0 x 108 m/s. The seven types, in order of decreasing wavelength (and increasing frequency), are: radio, microwave, infrared, visible light, ultraviolet, X-ray and gamma.

A short way to remember the order from the longest wavelength is: Really Mean Interesting Very Unusual X-ray Girls (the first letters: R, M, I, V, U, X, G).

the electromagnetic spectrum (longest wavelength on the left)radiomicrowaveinfraredvisibleUVX-raygammalonger wavelength, lower frequencyshorter wavelength, higher frequency
The electromagnetic spectrum: radio has the longest wavelength and gamma the shortest

Key ideas

  • Radio: longest wavelength, lowest frequency.
  • Gamma: shortest wavelength, highest frequency, most energy per photon.
  • All EM waves travel at 3.0 x 10^8 m/s in a vacuum.
  • Order: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.
Worked example 19.1.1
Which type of EM wave has the shortest wavelength? Write the name.

Method. Look at the end of the spectrum with the shortest wavelength.

  1. Look. The spectrum runs from radio (longest) to gamma (shortest).
    gamma at the end
  2. Decide. So gamma has the shortest wavelength.
    gamma
  3. Write the answer.
    gamma

Check. ✓

Worked example 19.1.2
Put these in order of increasing wavelength: X-ray, visible light, microwave. Write the three names.

Method. Use the order of the spectrum.

  1. Order. Visible light has a short wavelength; X-rays are shorter still; microwaves are the longest of the three.
    visible, X-ray, microwave
  2. Arrange. From shortest to longest: X-ray, visible light, microwave.
    X-ray, visible, microwave
  3. Write the answer.
    X-ray, visible light, microwave

Check. ✓

Worked example 19.1.3
Which type of EM wave has the highest frequency? Write the name.

Method. Look at the end of the spectrum.

  1. Look. Frequency increases towards the short-wavelength end.
    short wavelength
  2. Decide. Gamma rays have the highest frequency.
    gamma
  3. Write the answer.
    gamma

Check. ✓

Watch out

Watch out: the order is for wavelength. Longer wavelength means lower frequency. Radio waves have the lowest frequency; gamma the highest.

Exam tip

Exam tip. When asked to order the EM spectrum, start with radio at the long-wavelength end and finish with gamma at the short-wavelength end.

Exercise 19A  Order of the spectrum

1
FWhich type of EM wave has the longest wavelength? Write the name.
2
FWhich type of EM wave has the highest frequency? Write the name.
3
FWhich type of EM wave lies between microwave and visible light? Write the name.
4
FWhich type of EM wave lies between visible light and X-rays? Write the name.
5
FWhat is the speed of all electromagnetic waves in a vacuum, approximately? Write the number as 3.0 x 10 to the power of something, in m/s.
6
FWhich type of EM wave is used in a TV remote control? Write the name.
7
PSPut these in order from the longest wavelength to the shortest: gamma, radio, visible light. Write the three names.
8
PSWhich has the greater frequency: visible light or infrared? Write the name.
9
RExplain why all the EM waves travel at the same speed in a vacuum, in one sentence. Accept any sentence that says they are all electromagnetic waves with the same speed of light in a vacuum.
10
RA pupil says “gamma rays travel faster than radio waves in a vacuum.” Is the pupil right? Write yes or no.
11
PSArrange these in order of increasing frequency: radio, X-ray, infrared. Write the three names.
19.2

Uses of electromagnetic waves

GCSE
Describe uses of each type of electromagnetic wave
Video lesson: Uses of electromagnetic wavesComing soon
Context. A hospital uses one type of EM wave to see inside a patient, and a bank uses another to check banknotes. A mobile phone uses a third, and a remote control uses a fourth. Each use depends on how the wave interacts with the materials it meets.

Each type of EM wave has uses that match its properties. Radio waves carry broadcasts and communications. Microwaves carry satellite and phone signals and heat food. Infrared is used in remote controls, heaters and thermal cameras. Visible light is used for vision and for signals in optical fibres. Ultraviolet is used in fluorescent lamps and for security marking. X-rays are used in medical imaging and security scanners. Gamma rays are used to sterilise equipment and treat cancer, and as tracers.

TypeExample uses
radiobroadcasting and communication
microwavesatellite communications, mobile phones, cooking food
infraredremote controls, thermal imaging, heaters
visible lightseeing, optical fibres for communication
ultravioletfluorescent lamps, security marking, sterilising
X-raymedical imaging, security scanners
gammasterilising medical equipment, treating cancer, tracers
Uses of the seven types of electromagnetic wave

Key ideas

  • Radio: broadcasting, communication.
  • Microwave: satellites, phones, cooking.
  • Infrared: remote controls, thermal imaging, heaters.
  • X-ray and gamma: medical use, sterilising, tracers, cancer treatment.
Worked example 19.2.1
Which type of EM wave is used in medical X-ray imaging? Write the name.

Method. Recall the use.

  1. Look. Think about what the question is asking.
    look
  2. Recall. X-rays pass through soft tissue but are absorbed by bone, so they give an image.
    X-ray
  3. Write the answer.
    X-ray

Check. ✓

Worked example 19.2.2
A satellite dish receives signals from a satellite. Which type of EM wave is most likely used? Write the name.

Method. Recall the use of microwaves.

  1. Look. Think about what the question is asking.
    look
  2. Recall. Satellite communications use microwaves.
    microwave
  3. Write the answer.
    microwave

Check. ✓

Worked example 19.2.3
Which type of EM wave is used inside a fluorescent lamp to make the phosphor glow? Write the name.

Method. Recall the use.

  1. Look. The tube gives out a radiation that makes the phosphor coating glow.
    phosphor
  2. Recall. The radiation inside the tube is ultraviolet.
    ultraviolet
  3. Write the answer.
    ultraviolet

Check. ✓

Watch out

Watch out: a use depends on the wave, not on the object. X-rays are used for images because they pass through soft tissue, not because they are safe.

Exam tip

Exam tip. For a uses question, name the wave type, then give the property that makes it suitable (penetration, heating, reflection).

Exercise 19B  Uses of EM waves

1
FWhich type of EM wave is used to broadcast radio programmes? Write the name.
2
FWhich type of EM wave is used in a microwave oven? Write the name.
3
FWhich type of EM wave is used in a heater to warm a room? Write the name.
4
FWhich type of EM wave is used to sterilise medical equipment? Write the name.
5
FWhich type of EM wave is used in fibre-optic communication? Write the name.
6
FWhich type of EM wave is used in a fluorescent lamp? Write the name.
7
RWhy is infrared used in a thermal camera? Write one word: heat or colour.
8
PSWhich type of EM wave is used in a remote control to send a signal to a TV? Write the name.
9
PSWhich EM waves are used to carry broadcasts over long distances from a transmitter? Write the name.
10
RExplain why X-rays are used for bone scans but not for everyday lighting, in one sentence. Accept any sentence that says X-rays pass through soft tissue but are absorbed by bone, and are not suitable for lighting.
11
PSWhich two types of EM wave are used in communication? Write two names, separated by a comma.
12
RExplain why a satellite uses microwaves rather than visible light for communication, in one sentence. Accept any sentence that says microwaves pass through the atmosphere and clouds, so the signal is reliable.
19.3

Dangers of electromagnetic radiation

GCSE
Describe the dangers of ultraviolet, X-ray, gamma, microwave and infrared radiation, and how to reduce them
Video lesson: Dangers of electromagnetic radiationComing soon
Context. A lifeguard works on the beach all summer. The skin on her arms goes red after a long day. “What damaged it?” she asks. Ultraviolet from the Sun is the answer, and sunscreen and shade reduce the risk.

Some EM waves are dangerous. Ultraviolet can damage skin cells, causing sunburn and, over time, skin cancer. X-rays and gamma rays are ionising: they can knock electrons out of atoms and damage cells and DNA. Exposure is reduced by shielding (lead for X-rays and gamma), by limiting the time, and by keeping a large distance. Microwaves are absorbed by body tissue and can cause heating, which is why ovens have a shield. Infrared transfers heat, and can cause burns if the energy is too high.

TypeMain dangerWhy
ultravioletsunburn, skin damage and skin cancercan damage the skin cells; wear sunscreen and avoid long exposure
X-ray and gammadamage to cells and DNAthey are ionising, so they can change cells
microwaveheating of body tissuesthe wave is absorbed and heats the tissue (as in an oven)
infraredburns from heatthe wave transfers heat energy to the skin
Dangers of EM waves and why they are dangerous

Key ideas

  • Ultraviolet: skin damage and skin cancer; reduce with shade, sunscreen and clothing.
  • X-ray and gamma: ionising, so they damage cells; reduce with lead shielding, time and distance.
  • Microwave: heats tissue; ovens are shielded.
  • Infrared: heat, which can burn the skin.
Worked example 19.3.1
Why are X-rays and gamma rays more dangerous than radio waves? Write one reason in one sentence. Accept any sentence that says they are ionising and can damage cells.

Method. Think about the energy.

  1. Energy. X-rays and gamma rays have high frequency and high energy.
    high energy
  2. Ionising. So they are ionising and can damage cells, unlike radio waves.
    ionising
  3. Write the answer.
    Accept any sentence that says they are ionising and can damage cells.

Check. ✓

Worked example 19.3.2
Name the type of EM wave that causes sunburn. Write the name.

Method. Recall the wave.

  1. Look. Think about what the question is asking.
    look
  2. Recall. Ultraviolet light from the Sun causes sunburn.
    ultraviolet
  3. Write the answer.
    ultraviolet

Check. ✓

Worked example 19.3.3
Which ionising type of EM wave is used to sterilise medical equipment? Write the name.

Method. Think about the energy needed to kill microbes.

  1. Look. Sterilising needs a wave with a lot of energy, which is ionising.
    ionising
  2. Recall. Gamma rays are used to sterilise medical equipment.
    gamma
  3. Write the answer.
    gamma

Check. ✓

Watch out

Watch out: a wave is more dangerous as its frequency rises, because it carries more energy per photon. Radio waves are not the same as X-rays for safety.

Exam tip

Practical safety. Never look directly at the Sun or a bright light source, and never use lasers or strong UV lamps without supervision.

Exercise 19C  Dangers and safety

1
FWhich type of EM wave causes sunburn? Write the name.
2
FName a way to reduce the risk from ultraviolet from the Sun. Write one word.
3
FWhich type of EM wave is ionising and is used in hospitals to treat cancer? Write the name.
4
FWhat material is used to shield against X-rays and gamma rays? Write the name.
5
RWhy is a microwave oven fitted with a metal screen on the door? Write one word: heating or sound.
6
FWhich type of EM wave can cause burns through heating of the skin? Write the name.
7
RExplain why X-rays are ionising, in one sentence. Accept any sentence that says they have enough energy to knock electrons out of atoms.
8
PSName the type of EM wave that can pass through skin and be detected by a camera to see inside the body. Write the name.
9
RA pupil says “radio waves are the most dangerous type of EM wave.” Is the pupil right? Write yes or no.
10
RExplain in one sentence how the time and distance from a radioactive or X-ray source reduce the risk. Accept any sentence that says less time and more distance reduce the dose received.
11
RWhy should sunbeds be used with care? Write one reason in one sentence. Accept any sentence that says they expose the skin to ultraviolet, which can damage it.
19.4

The greenhouse effect (idea)

GCSE
Explain the greenhouse effect as the absorption and re-emission of infrared radiation by gases (idea)
Video lesson: The greenhouse effect (idea)Coming soon
Context. The Earth receives energy from the Sun as visible light and ultraviolet. The Earth gives out energy as infrared. Some gases in the air absorb this infrared, so the Earth stays warmer than it would be without them. This is the greenhouse effect.

Short-wavelength radiation from the Sun passes through the atmosphere and warms the Earth’s surface. The warm surface then emits infrared. Greenhouse gases such as carbon dioxide and methane absorb some of this infrared and re-emit it in all directions, so less energy escapes to space. The extra greenhouse gases from human activity increase this effect, which is linked to global warming.

Key ideas

  • The Sun gives out mainly visible light and ultraviolet; the Earth gives out infrared.
  • Greenhouse gases absorb infrared and re-emit it.
  • More greenhouse gas means more energy trapped, so the Earth warms.
  • Carbon dioxide and methane are greenhouse gases.
Worked example 19.4.1
Name a greenhouse gas that absorbs infrared. Write the name.

Method. Recall a greenhouse gas.

  1. Look. Think about what the question is asking.
    look
  2. Recall. Carbon dioxide is a greenhouse gas that absorbs infrared.
    carbon dioxide
  3. Write the answer.
    carbon dioxide

Check. Accept methane or water vapour. ✓

Worked example 19.4.2
Does the Earth give out mainly infrared or ultraviolet? Write the word.

Method. Think about the temperature of the Earth.

  1. Look. Think about what the question is asking.
    look
  2. Recall. A warm body gives out infrared radiation, so the Earth gives out mainly infrared.
    infrared
  3. Write the answer.
    infrared

Check. ✓

Worked example 19.4.3
Which greenhouse gas is released when fuels burn in cars and power stations? Write the name.

Method. Recall the gas.

  1. Look. Burning fuels releases a gas into the air.
    burning
  2. Recall. That gas is carbon dioxide, a greenhouse gas.
    carbon dioxide
  3. Write the answer.
    carbon dioxide

Check. ✓

Watch out

Watch out: the greenhouse effect is not the same as the ozone layer. Greenhouse gases trap infrared; the ozone layer absorbs most of the Sun’s harmful ultraviolet.

Exam tip

Exam tip. In an explanation of the greenhouse effect, say: infrared is absorbed and re-emitted by greenhouse gases, so less energy escapes.

Exercise 19D  The greenhouse effect

1
FWhat type of EM radiation does the Earth mainly give out? Write the name.
2
FWhich gas is a greenhouse gas that increases because of burning fossil fuels? Write the name.
3
FWhat do greenhouse gases do with infrared? Write one word: absorb or reflect.
4
FWhich type of EM wave does the Sun give out most strongly in the visible range? Write the name.
5
RExplain in one sentence how greenhouse gases warm the Earth. Accept any sentence that says they absorb and re-emit infrared, so less energy escapes to space.
6
RA pupil says the ozone layer is the same as the greenhouse effect. Is the pupil right? Write yes or no.
7
FWhich wave is absorbed by the ozone layer? Write the name.
8
RExplain why the Earth would be colder without greenhouse gases, in one sentence. Accept any sentence that says less infrared would be trapped, so more energy would escape to space.
9
FWhich type of EM wave is given out mainly by a warm object at room temperature? Write the name.
10
PSIs the frequency of microwaves lower or higher than that of infrared? Write lower or higher.
19.5

[H] Calculations with the speed of light

GCSE
[H] Use v = f lambda with c = 3.0 x 10^8 m/s to calculate the wavelength or frequency of an electromagnetic wave
Video lesson: [H] Calculations with the speed of lightComing soon

[H] Higher tier only. All EM waves travel at c = 3.0 x 108 m/s in a vacuum (and very nearly in air). So for any EM wave, c = f x lambda. Use this to find the wavelength from the frequency, or the frequency from the wavelength. Write the answers in standard form.

Key ideas

  • [H] c = f x lambda, with c = 3.0 x 10^8 m/s, f in Hz and lambda in m.
  • [H] lambda = c / f; f = c / lambda.
  • [H] Convert nm, um and cm to m before you calculate.
  • [H] Higher frequency means shorter wavelength.
Worked example 19.5.1
[H] A visible light wave has a frequency of 6.0 x 1014 Hz. Calculate its wavelength in m. Write the answer in standard form.

Method. Use lambda = c / f.

  1. Take the numbers. c = 3.0 x 108 m/s and f = 6.0 x 1014 Hz.
    c = 3.0 x 10^8, f = 6.0 x 10^14
  2. Divide. lambda = 3.0 x 108 / 6.0 x 1014 = 0.5 x 10-6 = 5.0 x 10-7 m.
    5.0 x 10^-7
  3. Write the answer.
    5.0 x 10^-7 m

Check. ✓

Worked example 19.5.2
[H] A microwave has a wavelength of 3 cm. Calculate its frequency in Hz. Write the answer in standard form.

Method. Convert to m, then use f = c / lambda.

  1. Convert. 3 cm = 0.03 m = 3 x 10-2 m.
    3 x 10^-2
  2. Divide. f = 3.0 x 108 / 3 x 10-2 = 1.0 x 1010 Hz.
    1.0 x 10^10
  3. Write the answer.
    1.0 x 10^10 Hz

Check. ✓

Worked example 19.5.3
[H] Calculate the wavelength of a wave of frequency 3.0 x 108 Hz. Write the number of metres.

Method. Use lambda = c / f.

  1. Divide. lambda = 3.0 x 108 / 3.0 x 108 = 1 m.
    1
  2. Check. c = f x lambda = 3.0 x 108 x 1 m, which is correct.
    1 m
  3. Write the answer.
    1 m

Check. ✓

Watch out

Watch out: [H] convert the wavelength to metres first. 3 cm is 0.03 m, and 500 nm is 5 x 10-7 m.

Exam tip

Exam tip. [H] Write c = f lambda, rearrange for the quantity you need, substitute with standard form, and check the answer is in the right range.

Exercise 19E  [H] Wavelength and frequency calculations

1
PS[H] Calculate the speed of a wave with f = 2 x 106 Hz and lambda = 150 m. Write the number of m/s.
2
PS[H] Calculate the wavelength of a wave of frequency 1 x 108 Hz. Write the number of metres.
3
PS[H] Calculate the frequency of a wave of wavelength 6 m. Write the answer in standard form.
4
PS[H] Which has the longer wavelength: a wave of frequency 1 x 109 Hz or one of frequency 1 x 1015 Hz? Write 1 x 10^9 or 1 x 10^15.
5
R[H] Explain in one sentence why the answer to c = f lambda is always about 3 x 10 to the 8 m/s for light in a vacuum. Accept any sentence that says this is the speed of light, which is the same for all EM waves in a vacuum.
6
PS[H] Calculate the frequency of a wave with wavelength 0.5 m. Write the answer in standard form.
7
PS[H] A wave has a frequency of 5.0 x 1014 Hz. Calculate its wavelength in m, in standard form.
8
PS[H] Calculate the wavelength of a radio wave of frequency 1.0 x 106 Hz. Write the number of metres.
9
PS[H] A microwave oven operates at 2.45 x 109 Hz. Calculate the wavelength in m to 2 significant figures.
10
PS[H] Which has the larger frequency: a wave of wavelength 1 cm or one of wavelength 1 m? Write 1 cm or 1 m.

Chapter summary

SkillMethod
Order of the spectrumRadio, microwave, infrared, visible, ultraviolet, X-ray, gamma (long to short wavelength)
SpeedAll EM waves travel at 3.0 x 10^8 m/s in a vacuum
Uses and dangersEach use depends on the wave; ultraviolet, X-ray and gamma are the most harmful
Greenhouse effectGreenhouse gases absorb and re-emit infrared, so less energy escapes
[H] Calculationsc = f lambda; lambda = c / f; f = c / lambda

Can you…?

  • I can order the seven types of EM wave
  • I can match each type to its uses
  • I can explain the dangers and how to reduce them
  • I can explain the greenhouse effect using infrared
  • [H] I can calculate wavelength and frequency using c = f lambda

Key terms

electromagnetic wave
a transverse wave that can travel through a vacuum, at the speed of light
ionising radiation
radiation that has enough energy to knock electrons out of atoms
greenhouse effect
the trapping of infrared by greenhouse gases in the atmosphere
ultraviolet
an EM wave with a shorter wavelength than visible light, which can damage skin
[H] speed of light (c)
the speed of EM waves in a vacuum, 3.0 x 10^8 m/s

Exercise 19Z  Check-up

1
Name the type of EM wave with the longest wavelength. Write the name.
2
Name the type of EM wave between microwave and visible light. Write the name.
3
What is the speed of EM waves in a vacuum, approximately? Write the number in m/s as 3 x 10 to the power of 8.
4
Which type of EM wave is used in airport security scanners and medical imaging? Write the name.
5
Which type of EM wave causes skin cancer over time? Write the name.
6
Name the greenhouse gas that absorbs infrared and is produced when fossil fuels burn. Write the name.
7
Name the material used to shield against gamma rays. Write the name.
8
Which type of EM wave is used in a remote control? Write the name.
9
Which type of EM wave is used in satellite communications? Write the name.
10
Explain why microwave ovens have a metal screen on the door, in one sentence. Accept any sentence that says microwaves would otherwise escape and heat people nearby.

Total 28 marks. Allow about 28 minutes.

Exercise 19X  Exam-style questions

1
FPut these in order from the shortest wavelength to the longest: radio, X-ray, infrared. Write the three names.[2 marks]
2
FName the type of EM wave used to: (a) carry radio broadcasts and (b) heat food in a microwave oven. Write two names, separated by a comma.[2 marks]
3
RExplain why gamma rays are more dangerous to the body than visible light, in two sentences. Accept any answer that says gamma rays are ionising and damage cells and DNA, whereas visible light is not ionising.[3 marks]
4
RDescribe the greenhouse effect, using the words infrared, absorb and re-emit. Accept any answer that says the Earth gives out infrared, greenhouse gases absorb and re-emit it, so less energy escapes and the Earth warms.[4 marks]
5
PS[H] A radio wave has a frequency of 2 x 106 Hz. Calculate its wavelength in m, in standard form. Write the number.[3 marks]
6
RDescribe two dangers of ultraviolet from the Sun, and one way to reduce the risk. Accept any answer that gives skin damage or sunburn, and skin cancer, and names sunscreen, shade or clothing.[3 marks]
7
RExplain why an X-ray image shows bones more clearly than soft tissue. Accept any answer that says X-rays are absorbed more by bone than by soft tissue, so bone casts a shadow on the detector.[3 marks]
8
PS[H] Calculate the frequency of a microwave with a wavelength of 0.12 m. Write the answer in standard form.[3 marks]
9
RCompare radio waves and X-rays, in terms of wavelength, frequency and one use of each. Accept any answer that gives radio as the longer wavelength and lower frequency, used for broadcasting, and X-rays as the shorter wavelength and higher frequency, used in medical imaging.[4 marks]
10
FName the process in which an atom loses an electron after absorbing high-energy radiation. Write the word.[1 mark]
← Chapter 18: Reflection, refraction and lensesChapter 20: Magnetism, electromagnets, the motor effect and induction →

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