What is an electromagnetic wave?

Electromagnetic (EM) waves are transverse waves, like ripples on water, but nothing physical goes up and down. What vibrates is the strength and direction of the electric and magnetic fields. A changing electric field creates a magnetic field, and a changing magnetic field creates an electric field, so each keeps the other going and the wave travels on by itself, even through a vacuum. That is how sunlight crosses 150 million kilometres of empty space to reach us. For the basics of waves in general, see our guide to what a wave is.

Properties of electromagnetic waves

• Transverse: the fields vibrate across the wave's path, not along it.
• No medium needed: they travel through a vacuum, unlike sound.
• Same speed in a vacuum: all EM waves travel at exactly 299,792,458 m/s, the speed of light. In AQA's GCSE words, all types travel at the same velocity through a vacuum (space) or air.
• They transfer energy from a source to an absorber.
• They can be reflected, refracted, diffracted and absorbed, like other waves. They slow down in materials such as glass and water, which is why they bend (refract).

The electromagnetic spectrum

The electromagnetic spectrum is the whole range of EM waves, in order of wavelength. Because they all travel at the same speed, a longer wavelength means a lower frequency, and a lower frequency means less energy per photon.

Exact boundaries vary between sources, because the bands blend into each other. NASA's tour of the electromagnetic spectrum covers each band with images from telescopes that observe in it. Visible light is a tiny slice, from about 380 nm (violet) to 700 nm (red).

A common memory aid for the order, from long to short wavelength, is "Raging Martians Invaded Venus Using X-ray Guns": radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.

Ionising and non-ionising radiation

High-frequency EM waves, from part of the ultraviolet band through X-rays to gamma rays, carry enough energy per photon to knock electrons out of atoms. This is ionising radiation, and it can damage DNA, which is why X-ray staff stand behind shields. Radio waves, microwaves, infrared and visible light are non-ionising: their photons don't have enough energy to do this, however bright the source.

Electromagnetic waves and photons

Light behaves as a wave, but it is also delivered in packets of energy called photons. Each photon's energy depends only on the frequency:

That is why ultraviolet can cause sunburn while bright red light can't: each UV photon carries more energy. Try a photon energy calculation, then press Mark to check it in our Light and Energy course.

Worked examples

Example 1: the wavelength of Wi-Fi

Wi-Fi often uses a frequency of 2.4 GHz, which is 2.4 × 10⁹ Hz. Its wavelength is λ = c ÷ f = 299,792,458 ÷ 2,400,000,000 = 0.125 m, about 12.5 cm. That places it in the microwave band.

Example 2: the frequency of red light

Red light has a wavelength of about 700 nm, which is 700 × 10⁻⁹ m. Its frequency is f = c ÷ λ = 299,792,458 ÷ (700 × 10⁻⁹) = 4.28 × 10¹⁴ Hz: over 400 trillion vibrations a second.

Example 3: comparing photon energies

Using E = hf = hc ÷ λ, a photon of green light (500 nm) has energy 3.97 × 10⁻¹⁹ J, while an X-ray photon of 0.1 nm has 1.99 × 10⁻¹⁵ J. The X-ray photon's wavelength is 5,000 times shorter, so it carries 5,000 times the energy, which is why X-rays are ionising and light isn't.

Example 4: how old is sunlight?

The Sun is about 1.496 × 10¹¹ m away. Light takes 1.496 × 10¹¹ ÷ 299,792,458 = 499 s to arrive, about 8 minutes 19 seconds. You always see the Sun as it was just over 8 minutes ago.

Who discovered electromagnetic waves?

In the 1860s, James Clerk Maxwell combined the laws of electricity and magnetism into one set of equations. In his 1865 paper, he showed they predicted waves of electric and magnetic fields travelling at a speed that matched the measured speed of light, and concluded that light itself is an electromagnetic wave. In the late 1880s, Heinrich Hertz made and detected radio waves in his laboratory, confirming the prediction. Within a few years, others were using them to send messages without wires.

Common mistakes

• Saying EM waves need a medium. They travel through a vacuum. Sound is the wave that needs a medium.
• Saying some EM waves travel faster. In a vacuum all of them travel at the same speed. They differ in wavelength and frequency.
• Calling EM waves longitudinal. They are transverse.
• Thinking brighter light is ionising. Ionisation depends on the energy of each photon, set by the frequency, not on the brightness.
• Mixing up wavelength and frequency order. Radio has the longest wavelength and lowest frequency. Gamma has the shortest wavelength and highest frequency.

Read next

Physics · Waves and lightRadio waves: what they are, uses and how they workRadio waves for GCSE Physics: their place in the electromagnetic spectrum, how aerials make and receive them, their uses, and wavelength calculations.