Heat, light and sound: waves in daily life

Heat, light, and sound behave very differently, but two of them — light and sound — are waves, and heat moves by three distinct processes. Heat travels by conduction, when vibrating particles pass energy to neighbours, as in a metal spoon warming in hot soup; by convection, when a fluid itself moves, as in warm air rising; and by radiation, which needs no medium at all, as in sunlight warming the Earth across empty space. Insulation works by blocking all three: trapping still air defeats conduction and convection, while reflective surfaces reduce radiation.

Light is an electromagnetic wave that travels through a vacuum at about 300,000 kilometres per second — the fastest speed known. When light meets a surface it may reflect, giving mirrors and the Moon's shine; refract, bending as it passes from air into water, which is why a straw in a glass looks broken; or be absorbed, warming the material. Lenses use refraction to focus light, which is how spectacles, cameras, and microscopes work. Colour is not a property painted onto objects: an object looks red because it reflects red wavelengths and absorbs others, and white light is a mixture of many wavelengths, as a prism shows.

Sound is a mechanical wave: vibrations travelling through a medium, usually air, as compressions and rarefactions. Unlike light, sound cannot travel through a vacuum, which is why space is silent in reality whatever films suggest. Sound travels at about 343 metres per second in air at room temperature, faster in water, and faster still in solids — which is why you can hear a distant train by putting an ear to a rail. Pitch depends on frequency, the number of vibrations per second, and loudness on amplitude. Ultrasound, at frequencies above human hearing, is used in medical imaging and in checking welds and machinery, because waves reflect from boundaries inside objects.

These ideas explain familiar technology. A microwave oven heats food by making water molecules vibrate, which is why it heats wet food faster than dry plates. Noise-cancelling headphones produce sound waves that cancel incoming ones. Fibre-optic cables carry light signals by total internal reflection, bouncing light along a glass thread with little loss. In each case, the application follows directly from the wave's behaviour, not from any special property of the device.