Waves and the electromagnetic spectrum
Physics · GCSE · The School
Describing a wave
Amplitude is the height (loudness or brightness); wavelength is the distance between peaks; frequency is waves per second in hertz. Wave speed = frequency × wavelength. Transverse waves vibrate across the direction of travel (light); longitudinal waves vibrate along it (sound), which is why sound needs a medium and light does not.
The spectrum in order
Radio, microwave, infrared, visible, ultraviolet, X-ray, gamma — longest wavelength to shortest, lowest frequency to highest, and lowest energy to highest. All travel at the speed of light in a vacuum. Learn the order once and every use and hazard question becomes answerable by position.
Uses and dangers track the energy
Radio: broadcasting. Microwave: cooking and phones. Infrared: heating and remote controls. Visible: sight and fibre optics. UV: sterilising, but it damages skin. X-rays: imaging bone, but ionising. Gamma: killing cancer cells, and dangerous for the same reason. The higher-energy end is hazardous precisely because it can ionise atoms in cells.
Wave speed = frequency × wavelength. Radio 4 broadcasts on 198 kHz long wave. Radio waves travel at 3 × 10⁸ m/s, so the wavelength is 3 × 10⁸ ÷ 198,000 = about 1,515 m — one and a half kilometres from crest to crest, which is why the transmitting mast has to be so tall. Now go the other way: green light has a wavelength of about 5 × 10⁻⁷ m, so its frequency is 3 × 10⁸ ÷ 5 × 10⁻⁷ = 6 × 10¹⁴ Hz.