Key Stage 4 science · Physics – Wave motion

GCSE electromagnetic waves: the spectrum, uses and hazards

The electromagnetic spectrum is one family of transverse waves that runs from radio waves to gamma rays. In Years 10 and 11, GCSE physics students learn the order of the waves, how their wavelengths and frequencies compare, how each type is produced and detected, what it is used for, and which kinds can harm the body and why.

The electromagnetic spectrum, explained

All electromagnetic waves are transverse waves that transfer energy, and they all travel at the same speed in a vacuum, about 3.0 × 10⁸ metres per second, which is the speed of light. That is why we can see the Sun and receive satellite signals across empty space. In order from the longest wavelength and lowest frequency, the spectrum is radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. Wavelengths range from radio waves of metres or more to gamma rays far smaller than an atom. Because the speed is fixed, a higher frequency means a shorter wavelength, according to the wave equation, and the higher-frequency waves carry more energy.

Students also learn how the waves are produced and detected. Radio waves are made when an alternating current flows in an electrical circuit and an aerial: the electrons oscillate and give out waves of the same frequency. When radio waves are absorbed by a receiving aerial, they produce an alternating current of that frequency. Changes in atoms and their nuclei can also produce or absorb electromagnetic waves across a wide range of frequencies. When an electron in an atom drops to a lower energy level, the atom gives out a wave, such as visible light or ultraviolet; when the atom absorbs a wave, an electron moves up a level. Gamma rays come from changes in the nucleus of an unstable atom. Detection includes aerials for radio waves, the retina in the eye, photographic film and electronic cameras for visible light and X-rays, and thermal cameras for infrared.

Each type of wave has its uses, which depend on how it behaves. Radio waves are used for broadcasting and communication. Microwaves carry satellite and mobile phone signals and, in ovens, are absorbed by water in food. Infrared is used in grills, thermal imaging and remote controls. Visible light is used in photography and in optical fibres, where it carries data. Ultraviolet is used in security marking and in fluorescent lamps. X-rays pass through soft tissue but are absorbed by bone, so they are used in medical imaging. Gamma rays are used to sterilise medical equipment and in some cancer treatments.

The hazards also vary. Microwaves can heat body cells and infrared can burn skin. Ultraviolet, X-rays and gamma rays are ionising radiation: they have enough energy to damage cells and DNA, which can lead to mutations and cancer. Ultraviolet damages skin and eyes, which is why sun cream and sunglasses matter. Students learn that the risk depends on the type, the dose and the exposure time, and that medical staff limit exposure to X-rays and gamma rays, while patients are given them only when the benefit outweighs the risk.

Words your child will hear

Electromagnetic wave: A transverse wave that can travel through a vacuum at the speed of light.
Spectrum: The whole range of electromagnetic waves, ordered by frequency or wavelength.
Frequency: The number of waves passing a point each second, in hertz (Hz).
Infrared: Waves just beyond red light, given off by warm objects and felt as heat.
Ultraviolet: Waves just beyond violet light, which can damage skin and eyes.
Ionising radiation: Radiation with enough energy to knock electrons out of atoms, which can damage cells. In the spectrum, this means ultraviolet, X-rays and gamma rays.
Aerial: A conductor that produces or detects radio waves through oscillating electric current.

What the National Curriculum says

The programme of study for Key Stage 4 says:

Students should be taught about electromagnetic waves, velocity in vacuum; waves transferring energy; wavelengths and frequencies from radio to gamma-rays
Students should be taught about the production and detection of electromagnetic waves, by electrical circuits, or by changes in atoms and nuclei
Students should be taught about uses of electromagnetic waves in the radio, microwave, infra-red, visible, ultra-violet, X-ray and gamma-ray regions, and their hazardous effects on bodily tissues

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

A worked example

A radio transmitter broadcasts at a frequency of 100 MHz (1.0 × 10⁸ Hz). Find the wavelength of the waves, then the wavelength of an X-ray of frequency 3.0 × 10¹⁸ Hz. Take the speed of electromagnetic waves in a vacuum as 3.0 × 10⁸ m/s, and say what the answers show.

Use the wave equation, wave speed = frequency × wavelength, and rearrange it for the wavelength. All electromagnetic waves travel at the same speed in a vacuum, and air gives almost the same value.
```
wavelength = speed ÷ frequency
```

Radio waves.

λ = (3.0 × 10⁸) ÷ (1.0 × 10⁸)
  = 3.0 m

X-rays. The frequency is much larger, so the wavelength will be much smaller.
```
λ = (3.0 × 10⁸) ÷ (3.0 × 10¹⁸)
  = 1.0 × 10⁻¹⁰ m
```
Compare. The radio wavelength is a few metres, about the size of a room. The X-ray wavelength is about the width of a single atom, around thirty billion times shorter.
Link to uses and hazards. High-frequency waves carry more energy, so X-rays can ionise and damage cells, while radio waves cannot. A radio wave and an X-ray are the same kind of wave, but they interact with matter very differently.

The radio waves have a wavelength of 3.0 m and the X-rays 1.0 × 10⁻¹⁰ m. Both travel at the same speed, but the X-rays have far higher frequency and energy.

Common mistakes and how to help

Thinking radio waves are sound waves

Radios produce sound, so students assume the signal that carries it is also sound.

What helps: Explain that radio waves are electromagnetic and travel through empty space, but sound needs a medium. The radio’s circuits turn the received signal into electrical vibrations and then into sound from the speaker.

Saying all electromagnetic waves are dangerous, or that “radiation” always means radioactive

Media coverage uses “radiation” for hazardous things only, but light, radio and infrared are also radiation.

What helps: Split the spectrum in two: non-ionising (radio to visible) and ionising (ultraviolet upwards). Then look at the dose. Sunlight is essential for life, yet too much UV causes sunburn and increases the risk of skin cancer.

Believing you can only get sunburn on hot days

People link sunburn with heat, which is infrared. Ultraviolet is separate, and it is not felt as warmth.

What helps: Remind your child that UV passes through thin cloud and is strong on bright days even when it is cool. Check for the UV forecast in the weather report and use shade, clothing and sun cream.

Imagining electromagnetic waves need air or another medium to travel

Sound and water waves do need one, and most examples children meet first are of that kind.

What helps: Ask how light reaches us from the Sun across empty space. Electromagnetic waves are disturbances in electric and magnetic fields, so they need no material to carry them.

How to help at home

See infrared with a phone camera. Point a phone camera at the front of a television remote while pressing a button. Many cameras show a faint purple glow from the infrared light, which your eyes cannot see. Try a few remotes and ask which wave type is being used.
Make a rainbow. Place a glass of water on a sunny windowsill, or use a fine spray of water with the sun behind you, and look for the spectrum on the wall. Never look at the Sun itself. Ask your child which end of the visible spectrum has the longest wavelength and which has the highest frequency.
Hunt for the spectrum around the house. Make a list of everything that uses electromagnetic waves: a radio, a wireless router, a microwave oven, a remote control, a grill, a lamp and, away from home, an X-ray at the dentist. Ask your child to put each into its place on the spectrum and say what the wave does.
Talk about sun safety. On a bright day, explain why UV matters even in cool weather. Look at the label on sun cream or sunglasses for the protection rating and discuss how the products work by absorbing or reflecting ultraviolet before it reaches the skin or eyes.
Order the waves with a number line. Write the seven types on separate cards, and ask your child to arrange them from longest wavelength to shortest, then from lowest frequency to highest, and then to match each to one use. Swap and see whether anyone can spot the mistakes.

Builds on

Key Stage 3: Water waves and sound waves
Key Stage 3: Hearing, microphones and ultrasound
Key Stage 3: Light waves and reflection
Key Stage 3: Mirrors, lenses, the eye and colour