The Electromagnetic Spectrum

The Electromagnetic Spectrum

The electromagnetic (EM) spectrum is the full family of electromagnetic waves, arranged in order of their wavelength and frequency. Although these waves seem very different in how we use them, they share important properties and form one continuous spectrum.

Key terms

Electromagnetic spectrum — the continuous range of all electromagnetic waves, from radio waves to gamma rays.

Transverse wave — a wave in which the oscillations are at right angles (perpendicular) to the direction of energy transfer.

Ionising radiation — radiation with enough energy to knock electrons out of atoms, which can damage living cells.

Shared properties of all EM waves

Every electromagnetic wave, no matter which region it belongs to, shares these key properties:

They are all transverse waves.
They all transfer energy from a source to a detector.
They can all travel through a vacuum (empty space) — they do not need a medium.
In a vacuum they all travel at the same speed, the speed of light, $c = 3 \times 10^{8}\ \text{m/s}$.
They obey the wave equation $v = f\lambda$, so for EM waves $c = f\lambda$.

Because $c$ is constant in a vacuum, frequency and wavelength are inversely related: as wavelength decreases, frequency increases.

Exam tip

Learn the order of the spectrum perfectly — it is asked almost every year. A common mnemonic running from longest wavelength to shortest is:

Raging Martians Invaded Venus Using X-ray Guns

(Radio, Microwave, Infrared, Visible, Ultraviolet, X-ray, Gamma).

The order of the spectrum

The seven regions, listed from longest wavelength / lowest frequency to shortest wavelength / highest frequency, are:

1. Radio waves — longest wavelength, lowest frequency
2. Microwaves
3. Infrared
4. Visible light
5. Ultraviolet
6. X-rays
7. Gamma rays — shortest wavelength, highest frequency

Radio waves can have wavelengths longer than a kilometre, while gamma rays have wavelengths smaller than the diameter of an atom. As wavelength falls across the spectrum, both the frequency and the energy of the waves rise. The high-energy end (UV, X-rays, gamma) is the most dangerous because these waves are ionising.

Visible light — the part we can see

Visible light is the only region our eyes can detect. It is a tiny slice of the whole spectrum, sitting between infrared and ultraviolet. White light is actually a mixture of colours, which a prism can split (disperse) into a spectrum.

In order of increasing frequency (and decreasing wavelength), the colours are:

Red, Orange, Yellow, Green, Blue, Indigo, Violet — remember ROY G BIV.

Red light has the longest wavelength and lowest frequency of visible light, and sits next to infrared.
Violet light has the shortest wavelength and highest frequency, and sits next to ultraviolet.

Uses and dangers of each region

Each region's uses depend on its properties. Lower-frequency waves (radio, microwave) are good for communication; higher-frequency waves carry more energy and can be used for heating, imaging or killing cells — but the most energetic are also the most harmful.

Real world

Optical fibres carry telephone and internet data as pulses of infrared or visible light. The signals travel at very high speed and are reflected along the inside of the glass fibre by total internal reflection, allowing huge amounts of data to be sent quickly.

Why the high-frequency end is dangerous

Ultraviolet, X-rays and gamma rays are ionising: they carry enough energy to knock electrons out of atoms in living cells. This can damage or mutate DNA, leading to cancer. The higher the frequency, the greater the energy and the greater the risk.

Ways we protect ourselves:

UV — wear sunscreen, sunglasses and protective clothing; limit time in strong sunlight and on sun beds.
X-rays — radiographers stand behind lead screens and patients are exposed only briefly; lead aprons shield parts of the body not being imaged.
Gamma rays — use thick lead or concrete shielding, keep your distance, and limit exposure time.

Watch out

A common exam mistake is to claim microwaves and radio waves are ionising — they are not. Only UV, X-rays and gamma rays are ionising. Microwaves are dangerous because they cause heating of tissue, not ionisation.

Worked example

A radio station broadcasts at a frequency of $9.0 \times 10^{7}\ \text{Hz}$. Calculate the wavelength of the radio waves in air.

Use $c = f\lambda$, so $\lambda = \dfrac{c}{f}$.

Quick recap

The EM spectrum, longest to shortest wavelength: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.
All EM waves are transverse, travel through a vacuum at $c = 3 \times 10^{8}\ \text{m/s}$, and obey $c = f\lambda$.
As wavelength decreases, frequency and energy increase.