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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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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?

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    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.

Show the solutionHide the solution
  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.

Practice questions

Have a go at each one before you open its answer.

  1. Question 1 Complete 2 marks

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

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    Model answer

    Ultraviolet.

    Mark scheme

    • Ultraviolet — 1 mark
    • Correct position in order — 1 mark
  2. Question 2 Use the diagram 3 marks

    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.
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    Model answer

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

    Mark scheme

    • A: infrared — 1 mark
    • B: ultraviolet — 1 mark
    • A common property — 1 mark
  3. Question 3 Calculate 3 marks

    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)

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    Model answer

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

    Mark scheme

    • Converts to 1.0 × 10⁸ Hz — 1 mark
    • Correct substitution — 1 mark
    • 3.0 m — 1 mark
  4. Question 4 State 2 marks

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

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    Model answer

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

    Mark scheme

    • Transverse waves — 1 mark
    • Same speed in a vacuum — 1 mark
  5. Question 5 Explain 3 marks

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

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    Model answer

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

    Mark scheme

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

Quick check

  1. Which has the longest wavelength?

    1. AGamma rays
    2. BX-rays
    3. CVisible light
    4. DRadio waves
    Show answerHide answer

    D: Radio waves

    Radio waves.

  2. Which has the highest frequency?

    1. AGamma rays
    2. BRadio waves
    3. CMicrowaves
    4. DInfrared
    Show answerHide answer

    A: Gamma rays

    Gamma rays.

  3. All electromagnetic waves are...

    1. Alongitudinal
    2. Btransverse
    3. Csound waves
    4. Dslow
    Show answerHide answer

    B: transverse

    Transverse.

  4. Between infrared and ultraviolet is...

    1. AX-rays
    2. Bmicrowaves
    3. Cvisible light
    4. Dgamma rays
    Show answerHide answer

    C: visible light

    The visible part.

  5. In a vacuum all EM waves travel at...

    1. Adifferent speeds
    2. Bzero speed
    3. Cthe speed of sound
    4. Dthe same speed
    Show answerHide answer

    D: the same speed

    3.0 × 10⁸ m/s.

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