The Electromagnetic Spectrum

In short: The electromagnetic spectrum is the family of waves that all travel at the speed of light: radio, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. From radio to gamma the wavelength gets shorter and the frequency and energy get higher, and UV, X-rays and gamma can damage cells.

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

$c = 3.0 \times 1 0^{8} m/s$

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 $c = f λ$ .

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 $c = 3 \times 1 0^{8}$ 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.
$c = f λ$ links frequency and wavelength for every EM wave.

Quiz

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Try 3 questions now

Which EM wave has the longest wavelength?
1. A Radio waves
2. B Microwaves
3. C X-rays
4. D Gamma rays
Show answer
A: Radio waves
Radio waves are at the long wavelength end of the spectrum.
Which EM wave has the highest frequency?
1. A Radio waves
2. B Visible light
3. C Ultraviolet
4. D Gamma rays
Show answer
D: Gamma rays
Gamma rays have the shortest wavelength and highest frequency.
In a vacuum, compared with radio waves, X-rays travel:
1. A Faster
2. B Slower
3. C At the same speed
4. D Not at all
Show answer
C: At the same speed
All EM waves travel at c in a vacuum.