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.
Refraction: bending at a boundary
A wave crossing into a material where it travels slower bends towards the normal — light entering glass from air is the standard example, which is why a straw in a glass of water looks bent at the surface. The frequency never changes, only the speed and the wavelength, and it is that change of speed that does the bending, not the material "grabbing" the wave.
Rearranging the wave equation, and the units trap
Wave speed = frequency × wavelength rearranges to frequency = speed ÷ wavelength or wavelength = speed ÷ frequency — write down which one the question wants before substituting. The recurring slip is units: a wavelength given in nanometres or centimetres must be converted to metres first, or an answer comes out a million times too big or too small and looks plausible enough to hand in unchecked.