The electromagnetic spectrum
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.
- 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, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays.
- 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.
Practice
In a vacuum, how do the speeds of radio waves, light and gamma rays compare?
All electromagnetic waves travel at the same speed in a vacuum, $3.0 \times 10^8\ \text{m/s}$.
Practice
Put these EM waves in order of wavelength, longest first.
Radio waves have the longest wavelength and gamma rays the shortest: radio, microwave, infrared, visible, UV, X-ray, gamma.
Practice
Going from radio waves toward gamma rays, the frequency:
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.
Practice
Match each wave to a common use.
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.
Practice
Too much of which wave causes sunburn and skin cancer?
Ultraviolet has enough energy to damage skin cells, causing sunburn and skin cancer.
You've got it
Key idea
- 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)