ASPhysicsCIERevision Notes7. Waves7.4 Electromagnetic SpectrumElectromagnetic Spectrum

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First teaching 2023

First exams 2025

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Electromagnetic Spectrum (CIE AS Physics)

Revision Note

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Katie M

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Katie M

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Physics

Properties of Electromagnetic Waves

  • Visible light is just one part of a much bigger spectrum: The Electromagnetic Spectrum
  • All electromagnetic waves have the following properties in common:
    • They are all transverse waves
    • They can all travel in a vacuum
    • They all travel at the same speed in a vacuum (free space) — the speed of light 3 × 108 ms-1

  • The speed of light in air is approximately the same

 

EM wave, downloadable AS & A Level Physics revision notes

Oscillating electric and magnetic fields in an electromagnetic wave

  • These transverse waves consist of electric and magnetic fields oscillating at right angles to each other and to the direction in which the wave is travelling (in 3D space)
  • Since they are transverse, all waves in this spectrum can be reflected, refracted, diffracted, polarised and produce interference patterns

Uses of electromagnetic waves

  • Electromagnetic waves have a large number of uses. The main ones are summarised in the table below

Tables of Electromagnetic Waves and their Uses

WAVE USE
Radio Communication (radio and TV)
Microwave

Heating food

Communication (WiFi, mobile phones, satellites)
Infrared

Remote controls

Fibre optic

Communication

Thermal imaging (medicine and industry)

Night vision

Heating or cooking things

Motion sensors (for security alarms)
Visible Seeing and taking photographs/videos
Ultraviolet

Security marking (fluorescence)

Fluorescent bulbs

Tanning beds
X Rays X-ray images (medicine, airport security, and industry)
Gamma Rays

Sterilising medical instruments

Treating cancer

Exam Tip

You will be expected to recall the common properties of all electromagnetic waves in an exam question, however the speed of light will be given on the data sheet.

Wavelengths of Electromagnetic Waves

  • The electromagnetic spectrum is arranged in a specific order based on their wavelengths or frequencies
  • This order is shown in the diagram below from longest wavelength (lowest frequency) to shortest wavelength (highest frequency) 

Wavelengths across the EM Spectrum

EM spectrum diagram, downloadable AS & A Level Physics revision notes

Energy, wavelength and frequency for each part of the electromagnetic spectrum

  • The higher the frequency, the higher the energy of the radiation
  • Radiation with higher energy is highly ionising and is harmful to cells and tissues causing cancer (e.g. UV, X-rays, Gamma rays)
  • The approximate wavelengths in a vacuum of each radiation is listed in the table below:

 

EM spectrum wavelengths and frequencies

Estimating Physical Quantities Table

RADIATION WAVELENGTH RANGE / m
Radio > 0.1
Microwaves 0.1 to 1 × 10−3
Infrared 1 × 10−3 to 7 × 10−7
Visible 7 × 10−7 to 4 × 10−7
Ultraviolet 4 × 10−7 to 1 × 10−8
X Rays 1 × 10−8 to 4 × 10−13
Gamma Rays 4 × 10−13 to 10−16

  • To alternatively find the range of frequencies, convert the wavelengths using the wave equation: c = fλ where c is the speed of light: 3.0 × 108 m s−1
  • Electromagnetic radiation wavelengths of 400 nm to 700 nm are visible to the human eye

Worked example

A is a source emitting microwaves and B is a source emitting X-rays. The table suggests the frequencies for A and B. Which row is correct?

  Frequency emitted by A / Hz Frequency emitted by B / Hz
A 3 × 109 to 3 × 1011 > 1019
B 1 × 1012 to 1 × 1013 3 × 1016 to 7.5 × 1020
C 3 × 109 to 3 × 1011 3 × 1016 to 7.5 × 1020
D 4 × 1014 to 8 × 1014 5 × 1013 to 7 × 1015

Answer: C

Step 1: Recall the wavelength ranges of microwaves and X rays:

  • The wavelength range of microwaves is 0.1 m to 1 × 10−3 m
  • The wavelength range of X rays is 1 × 10−8 m to 4 × 10−13 m

Step 2: Apply the wave equation to find frequency ranges:

  • The speed of electromagnetic radiation is c , so to calculate frequency f use

f space equals space c over lambda

  • The frequency range of microwaves is

f subscript l o w end subscript space equals space fraction numerator 3.0 space cross times space 10 to the power of 8 over denominator 0.1 end fraction space equals space 3.0 space cross times space 10 to the power of 9 space Hz

f subscript h i g h end subscript space equals space fraction numerator 3.0 space cross times space 10 to the power of 8 over denominator 1 space cross times space 10 to the power of negative 3 end exponent end fraction space equals space 3.0 space cross times space 10 to the power of 11 space Hz

  • This eliminates B and D because source A emits microwaves
  • The frequency range of X rays is

f subscript l o w end subscript space equals space fraction numerator 3.0 space cross times space 10 to the power of 8 over denominator 1 space cross times space 10 to the power of negative 8 end exponent end fraction space equals space 3.0 space cross times space 10 to the power of 16 space Hz

f subscript h i g h end subscript space equals space fraction numerator 3.0 space cross times space 10 to the power of 8 over denominator 4 space cross times space 10 to the power of negative 13 end exponent end fraction space equals space 7.5 space cross times space 10 to the power of 20 space Hz

  • Both frequency ranges match option C

Exam Tip

You will be expected to memorise the range of wavelengths for each type of radiation, however you don’t need to learn the frequency ranges by heart. Since all EM waves travel at the speed of light, you can convert between frequency and wavelength using the wave equation in an exam question.

Visible Light

  • Visible light is defined as the range of wavelengths (400 - 700 nm) which are visible to humans
  • Visible light is the only part of the spectrum detectable by the human eye
    • However, this is only 0.0035% of the whole electromagnetic spectrum

  • In the natural world, many animals, such as birds, bees and certain fish, are able to perceive beyond visible light and can see infra-red and UV wavelengths of light

 

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    Katie M

    Author: Katie M

    Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.