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Physics · Waves

The electromagnetic spectrum

Name the groups of electromagnetic waves in order of wavelength and frequency, and describe their common properties.

  • 6 key terms
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Teacher resources

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Student handouts

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Warm-up

Answer each one, then check.

  1. 1

    What is a wave?

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    A way of transferring energy

  2. 2

    What is frequency?

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    Number of waves per second

  3. 3

    Name a colour of the rainbow.

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    Red, orange, yellow, green, blue, indigo or violet

  4. 4

    What is the speed of light in a vacuum?

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    \(3.0 \times 10^{8}\) m/s

  5. 5

    What type of wave is light?

    Show answerHide answer

    Transverse

Learning Objectives

  1. 1List the seven types of electromagnetic wave in order.
  2. 2State that all EM waves are transverse and travel at the same speed in a vacuum or air.
  3. 3Relate wavelength to frequency across the spectrum.
  4. 4Give examples of energy transfer by EM waves.

THE EM SPECTRUM

Electromagnetic waves are transverse waves that transfer energy from a source to an absorber. They form a continuous spectrum and all travel at the same speed through a vacuum or air.

From long to short wavelength (or low to high frequency): radio, microwave, infrared, visible light (red to violet), ultraviolet, X-rays, gamma rays.

The Seven Groups

Learn the order.

  • Radio waves

    Wavelength: Longest. Frequency: Lowest

  • Microwaves

  • Infrared

  • Visible light (red to violet)

  • Ultraviolet

  • X-rays

  • Gamma rays

    Wavelength: Shortest. Frequency: Highest

Using the Wave Equation

A radio wave has a frequency of 100 MHz. Calculate its wavelength. Speed of EM waves = 3.0 × 10⁸ m/s.

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  1. 1 Convert \(100\) MHz \(= 1.0 \times 10^{8}\) Hz
  2. 2 Rearrange \(\lambda = v \div f\)
  3. 3 Substitute \(\lambda = 3.0 \times 10^{8} \div 1.0 \times 10^{8}\)
  4. 4 Answer \(\lambda = 3.0\) m

Answer3.0 m

Memory Aid

Give a way to remember the order of the EM spectrum.

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  1. 1 Initials Radio, Microwave, Infrared, Visible, Ultraviolet, X-ray, Gamma
  2. 2 Sentence Rabbits Mate In Very Unusual eXpensive Gardens

AnswerUse a mnemonic such as 'Rabbits Mate In Very Unusual eXpensive Gardens'.

Energy Transfer

Give examples that show EM waves transfer energy from a source to an absorber.

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  1. 1 Microwaves Heat food
  2. 2 Infrared Warm your skin from a fire
  3. 3 Light Provides energy to plants and to solar cells
  4. 4 Gamma rays Transfer energy to cells

AnswerEach wave carries energy from its source to an absorber, where it increases a thermal or other energy store.

Key Facts

Remember these.

  • Transverse

    All EM waves are transverse.

  • Same speed

    They all travel at 3.0 × 10⁸ m/s in a vacuum.

  • Continuous

    There are no gaps between the groups.

  • Our eyes

    Detect only visible light, a limited range of the spectrum.

Order the Waves

Put these in order of increasing frequency: X-rays, radio waves, visible light, microwaves, gamma rays, infrared, ultraviolet.

1. Use the mnemonic.

A good answer shows: Radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays.

Can I...?

  1. 1List the EM spectrum in order.
  2. 2Say they are transverse.
  3. 3State the common speed.
  4. 4Link wavelength and frequency.
  5. 5Use \(v = f\lambda\) for EM waves.
  6. 6Name the visible colours.
  7. 7Say eyes detect only visible light.
  8. 8Give an example of energy transfer.

Summary & Exam Focus

  • Order: radio, microwave, infrared, visible, ultraviolet, X-rays, gamma.
  • All are transverse and travel at the same speed in a vacuum.
  • Wavelength decreases as frequency increases.
  • Eyes detect only visible light.

Exam focus

List the electromagnetic waves in order of increasing frequency, starting with radio waves. (2 marks) (2 marks)

Radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.

Key terms

The vocabulary this lesson expects you to use. Each one is linked from the first place it appears above.

Electromagnetic wave
A transverse wave that can travel through a vacuum.
Spectrum
A continuous range of wavelengths or frequencies.
Radio wave
The electromagnetic wave with the longest wavelength.
Gamma ray
The electromagnetic wave with the shortest wavelength.
Visible light
The part of the spectrum our eyes detect.
Absorber
An object that takes in the energy of a wave.

Questions and answers

10 questions set on this lesson, with the mark schemes and model answers open.

1. Exam question Complete 2 marks Easier

The electromagnetic spectrum is shown in order of increasing frequency. Radio waves, microwaves, infrared, visible light, ___, X-rays, gamma rays. Name the missing wave.

Mark scheme — 2 marks available

  • Ultraviolet — 1 mark
  • Correct position in order — 1 mark

Model answer

Ultraviolet.

2. Exam question Use the diagram 3 marks Easier

The diagram shows the electromagnetic spectrum with some waves missing. (a) Name the waves labelled A and B. (b) State one property that all electromagnetic waves have.

The electromagnetic spectrum with two sections, A and B, left blank.

Mark scheme — 3 marks available

  • A: infrared — 1 mark
  • B: ultraviolet — 1 mark
  • A common property — 1 mark

Model answer

(a) A: infrared. B: ultraviolet. (b) They are transverse, or they travel at the same speed in a vacuum.

3. Exam question Calculate 3 marks Easier

A radio wave has a frequency of 100 MHz. The speed of electromagnetic waves in air is 3.0 × 10⁸ m/s. Calculate the wavelength. (1 MHz = 10⁶ Hz)

Mark scheme — 3 marks available

  • Converts to 1.0 × 10⁸ Hz — 1 mark
  • Correct substitution — 1 mark
  • 3.0 m — 1 mark

Model answer

\(\lambda = 3.0 \times 10^8 \div 1.0 \times 10^8 = 3.0\) m

4. Exam question State 2 marks Easier

State two properties that are the same for all electromagnetic waves.

Mark scheme — 2 marks available

  • Transverse waves — 1 mark
  • Same speed in a vacuum — 1 mark

Model answer

They are all transverse. They all travel at the same speed in a vacuum (or air).

5. Exam question Explain 3 marks Easier

Gamma rays have a much higher frequency than radio waves. Explain why their wavelength is much shorter.

Mark scheme — 3 marks available

  • Same speed — 1 mark
  • v = fλ — 1 mark
  • Higher frequency means shorter wavelength — 1 mark

Model answer

All electromagnetic waves travel at the same speed in a vacuum. Since v = fλ, a greater frequency means a shorter wavelength.

6. Multiple choice 1 mark Easier

Which has the longest wavelength?

  1. A Gamma rays
  2. B X-rays
  3. C Visible light
  4. D Radio waves Correct

Why: Radio waves.

7. Multiple choice 1 mark Core

Which has the highest frequency?

  1. A Gamma rays Correct
  2. B Radio waves
  3. C Microwaves
  4. D Infrared

Why: Gamma rays.

8. Multiple choice 1 mark Core

All electromagnetic waves are...

  1. A longitudinal
  2. B transverse Correct
  3. C sound waves
  4. D slow

Why: Transverse.

9. Multiple choice 1 mark Core

Between infrared and ultraviolet is...

  1. A X-rays
  2. B microwaves
  3. C visible light Correct
  4. D gamma rays

Why: The visible part.

10. Multiple choice 1 mark Stretch

In a vacuum all EM waves travel at...

  1. A different speeds
  2. B zero speed
  3. C the speed of sound
  4. D the same speed Correct

Why: 3.0 × 10⁸ m/s.