The electromagnetic spectrum
Introduced| English | Meaning |
|---|---|
| electromagnetic spectrum/ɪˌlektrəʊməɡˈnetɪk ˈspektrəm/ | The complete range of electromagnetic radiation, ordered by frequency or wavelength. |
| radio waves/ˈreɪdɪəʊ weɪvz/ | Longest-wavelength electromagnetic waves, used for broadcasting and communication. |
| microwaves/ˈmaɪkrəʊweɪvz/ | Electromagnetic radiation in the microwave region, between radio waves of longer wavelength and infrared radiation. |
| infrared/ˌɪnfrəˈred/ | Electromagnetic radiation with wavelengths longer than visible red light and shorter than microwaves. |
| ultraviolet/ˌʊltrəˈvaɪəlɪt/ | Electromagnetic radiation with wavelengths shorter than visible violet light and longer than X-rays. |
| X-rays/eks reɪz/ | High-frequency electromagnetic waves used for medical imaging; ionising and hazardous. |
| gamma rays/ˈɡæmə reɪz/ | Electromagnetic radiation from nuclear transitions; compared with alpha and beta radiation, it is usually more penetrating and less ionising. |
One family of waves
- Radio, the warmth of a heater, the light you see, the X-ray at a hospital — all the same kind of wave.
- Together they make the electromagnetic spectrum 电磁波谱.
- They all travel at the same high speed in a vacuum: $3.0 \times 10^8\ \text{m/s}$.
The order of the spectrum
- From longest wavelength to shortest: radio waves 无线电波, microwaves 微波, infrared 红外线, visible light, ultraviolet 紫外线, X-rays X射线, gamma rays 伽马射线.
- As you go that way: the wavelength falls, the frequency rises, and the energy rises.
- Visible light is just a thin slice in the middle.
The electromagnetic spectrum
Slide across the spectrum
Radio waves, visible light and gamma rays are all the same wave — only the wavelength changes, and with it the frequency, photon energy and everyday use.
In a vacuum, how do the speeds of radio waves, light and gamma rays compare?
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All electromagnetic waves travel at the same speed in a vacuum, $3.0 \times 10^8\ \text{m/s}$.
Put these EM waves in order of wavelength, longest first.
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Radio waves have the longest wavelength and gamma rays the shortest: radio, microwave, infrared, visible, UV, X-ray, gamma.
Going from radio waves toward gamma rays, the frequency:
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As wavelength falls (radio → gamma), the frequency and the energy both rise.
What each one is used for
- Radio waves — radio and TV broadcasts.
- Microwaves — mobile phones, satellite TV, cooking.
- Infrared — remote controls, thermal imaging, grills.
- Visible light — seeing and photography.
- Ultraviolet — sterilising water, checking banknotes.
- X-rays — medical and security scans.
- Gamma rays — sterilising equipment, treating cancer.
A glass prism splits white light into the colours of the spectrum
Match each wave to a common use.
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Microwaves carry phone signals, infrared works remote controls, and X-rays image bones.
The dangers
- The shorter, higher-energy waves are the dangerous ones.
- Microwaves — heat the water in body cells.
- Infrared — skin burns.
- Ultraviolet — skin cancer and eye damage.
- X-rays and gamma rays — damage cells and cause mutations.
Too much of which wave causes sunburn and skin cancer?
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Ultraviolet has enough energy to damage skin cells, causing sunburn and skin cancer.
Worked examples
1. Wi-Fi uses microwaves of frequency $2.4\ \text{GHz} = 2.4 \times 10^9\ \text{Hz}$. Wavelength $\lambda = \dfrac{v}{f} = \dfrac{3.0 \times 10^8}{2.4 \times 10^9} = 0.125\ \text{m}$, about $12.5\ \text{cm}$.
2. Which has the higher frequency, ultraviolet or X-rays? X-rays: they come after ultraviolet in the order radio → microwave → infrared → visible → ultraviolet → X-ray → gamma.
All electromagnetic waves travel at the same speed in a vacuum, $3.0 \times 10^8\ \text{m/s}$. Gamma rays are not faster than radio waves; they have a higher frequency and a shorter wavelength.
You've got it
- all EM waves travel at $3.0 \times 10^8\ \text{m/s}$ in a vacuum
- order (long → short wavelength): radio · micro · infrared · visible · UV · X-ray · gamma
- going that way: frequency and energy rise, wavelength falls
- higher energy → more dangerous (UV cancer, X/gamma cell damage)
Write from a radio frequency of 1.0 times 10 to the 8 hertz
- A radio wave in a vacuum has wavelength 3.0 m. The vacuum speed is 3.0 times 10 to the 8 metres per second.
- Find the frequency. Gamma radiation does not travel faster in a vacuum.
A radio wave of wavelength 3.0 m travels in a vacuum at 3.0 times 10 to the 8 metres per second. What is its frequency, and how does that speed compare with gamma radiation?
3.0 times 10 to the 8 divided by 3.0 is 1.0 times 10 to the 8 hertz.
What is the frequency of this radio wave?
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Speed divided by wavelength is 1.0 times 10 to the 8 hertz.