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What are the bands of the electromagnetic spectrum, and how are they used and detected?

The electromagnetic spectrum: the order of the bands by wavelength and frequency, that all travel at the speed of light, and the uses, sources and detectors of each band including the dangers of the high-energy bands.

An SQA National 5 Physics answer on the electromagnetic spectrum, covering the order of the seven bands by wavelength and frequency, that all travel at the speed of light in a vacuum, the relationship v equals f times wavelength applied to light, and the uses, sources, detectors and dangers of each band.

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  1. What this key area is asking
  2. The order of the spectrum
  3. All travel at the speed of light
  4. Uses, sources and detectors
  5. The dangers of high-energy radiation
  6. Try this

What this key area is asking

The SQA wants you to know the order of the bands of the electromagnetic spectrum, that all the bands travel at the speed of light, the relationship v=fλv = f\lambda applied to light, and the uses, sources, detectors and dangers of each band.

The order of the spectrum

Radio waves have the longest wavelength and lowest frequency; gamma rays have the shortest wavelength and highest frequency. Visible light, the only band our eyes can detect, sits in the middle, with red at the long-wavelength end and violet at the short-wavelength end.

All travel at the speed of light

Because every band travels at the same speed in a vacuum, a higher frequency must go with a shorter wavelength, and the wave relationship lets you convert between them. The speed of light is enormous, so even high-frequency waves have very short wavelengths.

Uses, sources and detectors

Each band has characteristic uses and detectors that the SQA expects you to know:

  • Radio waves: broadcasting TV and radio; detected by an aerial.
  • Microwaves: mobile phones, satellite communication and cooking; detected by an aerial or a microwave receiver.
  • Infrared: remote controls, heaters and thermal (heat) imaging; detected by a thermometer, the skin or an infrared camera.
  • Visible light: seeing and optical communication (fibre optics); detected by the eye, photographic film and a photodiode.
  • Ultraviolet: security marking, sterilising and causing a suntan; detected by fluorescent materials and photographic film.
  • X-rays: medical imaging of bones and airport security; detected by photographic film and detectors.
  • Gamma rays: sterilising equipment and treating cancer; detected by a Geiger-Muller tube.

The dangers of high-energy radiation

The higher-frequency bands carry more energy. Ultraviolet can damage skin and eyes and cause skin cancer; X-rays and gamma rays are ionising and can damage living cells, which is why their use in medicine is carefully controlled. The lower-frequency bands carry less energy and are generally safer, though strong microwave or infrared sources can still cause heating.

Try this

Q1. List the bands of the electromagnetic spectrum in order of increasing frequency. [2 marks]

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

Q2. State the speed of all electromagnetic waves in a vacuum. [1 mark]

  • Cue. 3.0×108 m s−13.0 \times 10^8 \text{ m s}^{-1} (the speed of light).

Q3. Name the band used in a TV remote control. [1 mark]

  • Cue. Infrared.

Exam-style practice questions

Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

SQA N5 style3 marksA radio station broadcasts at a frequency of 100 MHz. Calculate the wavelength of the radio waves. The speed of light is 3.0 times 10 to the power 8 m per second.
Show worked answer →

All electromagnetic waves travel at the speed of light, so use v=fλv = f\lambda rearranged for wavelength. Note 100 MHz=100×106 Hz=1.0×108 Hz100 \text{ MHz} = 100 \times 10^6 \text{ Hz} = 1.0 \times 10^8 \text{ Hz}.

Relationship: λ=vf\lambda = \dfrac{v}{f}.

Substitution: λ=3.0×1081.0×108=3.0 m\lambda = \dfrac{3.0 \times 10^8}{1.0 \times 10^8} = 3.0 \text{ m}.

Markers reward converting the frequency to hertz, rearranging v=fλv = f\lambda, and a final answer in metres.

SQA N5 style3 marksName the band of the electromagnetic spectrum used for each of the following: heating food in a microwave oven, taking a photograph of broken bones, and a TV remote control.
Show worked answer →

Heating food in a microwave oven uses microwaves.

Taking a photograph of broken bones uses X-rays.

A TV remote control uses infrared.

Markers reward each correct band matched to its use. These are standard uses the SQA expects you to know for the bands of the spectrum.

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