Radio waves

Radio waves are the type of electromagnetic wave with the longest wavelengths and the lowest frequencies. Like all electromagnetic waves they are transverse and travel through a vacuum at the speed of light, 3.0 × 10⁸ m/s. They are made by alternating currents in aerials, and they carry radio, television and other communication signals.

Radio waves in the electromagnetic spectrum

AQA's GCSE specification lists the groups of the electromagnetic spectrum in order: radio, microwave, infrared, visible light (red to violet), ultraviolet, X-rays and gamma rays. Radio waves are at the long-wavelength, low-frequency end. All types of electromagnetic wave travel at the same velocity through a vacuum or air.

OpenStax University Physics describes radio waves as having wavelengths greater than about 0.1 m, though there is no sharp boundary with microwaves. Some radio wavelengths are many kilometres long.

How radio waves are made and received

AQA's specification states that radio waves can be produced by oscillations in electrical circuits. An alternating current in the wires of a transmitting aerial makes the electrons in it move back and forth, and that produces a radio wave with the same frequency as the current.

It works the other way round too. AQA puts it this way: when radio waves are absorbed, they may create an alternating current that oscillates at the frequency of the wave itself. A radio or television picks up that tiny current and turns the signal it carries back into sound or pictures. This is why aerials are made of metal: it contains free electrons that the wave can move.

Uses of radio waves

AQA lists television and radio as uses of radio waves. Other uses rely on the same properties:

• Radio and television broadcasting. Long wavelengths spread round obstacles and travel long distances, so one transmitter can cover a large area.
• Two-way radios and communications for emergency services, aircraft and ships.
• Radio astronomy. Astronomers detect radio waves given off by objects in space.
• Aerials that match the signal. Radio waves can be polarised, so a receiving aerial works best lined up with the transmitter's, as the polarisation guide explains.

Radio waves pass through the atmosphere easily. AQA's list of electromagnetic waves with hazardous effects on body tissue covers ultraviolet, X-rays and gamma rays, not radio waves.

Radio wave calculations

All radio waves obey the wave equation, with the wave speed equal to the speed of light:

Example 1: an FM radio wavelength

OpenStax gives the FM radio band as 88 to 108 MHz. Find the wavelength of a 100 MHz signal.

λ = v ÷ f = 3.0 × 10⁸ ÷ 1.00 × 10⁸ = 3.0 m.

Example 2: a long-wave frequency

A long-wave radio signal has a wavelength of 1500 m. Find its frequency.

f = v ÷ λ = 3.0 × 10⁸ ÷ 1500 = 200 000 Hz = 200 kHz.

Example 3: a signal from a satellite

A radio signal travels 36 000 km from a satellite to the ground. How long does it take?

t = distance ÷ speed = 3.6 × 10⁷ ÷ 3.0 × 10⁸ = 0.12 s.

Radio waves vs sound waves

A radio uses radio waves to carry a signal, and the speaker turns it into sound. More on electromagnetic waves in general is in what is an electromagnetic wave.

Common mistakes

• Calling radio waves sound waves. They are electromagnetic waves; sound is a mechanical wave.
• Getting the order wrong. Radio waves have the longest wavelength and the lowest frequency.
• Forgetting unit prefixes. MHz is 10⁶ Hz and kHz is 10³ Hz; convert before using v = fλ.
• Saying radio waves are ionising. They aren't: only ultraviolet, X-rays and gamma rays are listed as hazardous to body tissue.
• Saying the receiving aerial reflects the wave. It absorbs it, which induces an alternating current.

The electromagnetic spectrum is covered in the Light and Energy course. For practice in a short timed paper, try today's GCSE Physics Higher or GCSE Physics Foundation paper.

Read next

Physics · Waves and lightWhat is an electromagnetic wave? Spectrum, uses, examplesWhat an electromagnetic wave is, how it travels, the electromagnetic spectrum from radio to gamma rays, uses and dangers, and worked examples.