What are electromagnetic waves?
Electromagnetic (EM) waves are made of changing electric and magnetic fields. All EM waves:
- are transverse waves
- can travel through a vacuum (empty space). They do not need a medium
- travel at the same speed in a vacuum: the speed of light
- carry energy
- can be reflected, refracted and absorbed
The only real difference between them is their wavelength (and so their frequency).
The spectrum, in order
From longest wavelength (lowest frequency, lowest energy) to shortest wavelength (highest frequency, highest energy):
| Wave | Typical wavelength | Uses | Dangers |
|---|---|---|---|
| Radio waves | 1 m to many km | Radio, TV broadcasts, communication | Very low risk |
| Microwaves | 1 mm to 1 m | Cooking, mobile phones, Wi-Fi, satellites, radar | Heating of body tissue in large doses |
| Infrared (IR) | about 0.001 mm | Remote controls, heaters, thermal cameras, night vision | Burns |
| Visible light | 400 to 700 nm | Seeing, photography, optical fibres | Very bright light damages eyes |
| Ultraviolet (UV) | 10 to 400 nm | Sun tanning, sterilising water, security marks, makes vitamin D | Sunburn, skin cancer, eye damage |
| X-rays | about 0.01 to 10 nm | Medical images of bones, airport security | Can damage cells and cause cancer |
| Gamma rays | less than 0.01 nm | Killing cancer cells, sterilising medical tools, food | Most dangerous: damages cells and DNA |
Ionising radiation
Ultraviolet, X-rays and gamma rays carry enough energy to knock electrons out of atoms. This is called ionising radiation. It can damage DNA inside cells, which can cause mutations and cancer.
Protection:
- Sunscreen, hats and sunglasses block UV. Australia has one of the highest skin cancer rates in the world, so "Slip, Slop, Slap, Seek, Slide" really matters.
- Radiographers stand behind lead screens when taking X-rays.
- Gamma sources are stored in thick lead or concrete containers.
Using the wave equation
All EM waves obey .
EM spectrum
All EM waves
- Transverse
- Travel in a vacuum
- Speed c = 3 × 10⁸ m/s
Low energy end
- Radio: communication
- Microwaves: cooking, phones
- Infrared: heat, remotes
Middle
- Visible light
- ROY G BIV
High energy end
- Ultraviolet: sunburn
- X-rays: bones
- Gamma: cancer treatment
Pattern
- Shorter wavelength
- Higher frequency
- More energy, more danger
Quick summary
- EM waves are transverse, travel through a vacuum, and all move at m/s.
- Order (long to short wavelength): radio, microwaves, infrared, visible, ultraviolet, X-rays, gamma.
- Shorter wavelength means higher frequency and more energy.
- UV, X-rays and gamma are ionising and can damage cells.
- Every type has important uses, from Wi-Fi to cancer treatment.
- links frequency and wavelength for every EM wave.
Quiz
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