Waves, light and sound

Energy

提示Not the water itself — a duck sitting on a ripple bobs but stays put.

为什么A wave moves energy without carrying matter along with it. The water in a ripple, the air in a sound and the fibres in a shaken rope all wobble about a fixed spot while the wave travels on.

Vibrating

提示Touch a loudspeaker cone while music is playing and you can feel it.

为什么Every sound begins with something shaking — a guitar string, a drum skin, your vocal folds — and that shaking passes into the air. Damp the movement with your hand and the sound stops instantly.

It gets higher

提示Tighten a guitar string and listen to what the note does.

为什么Frequency counts vibrations per second in hertz, and our ears report that as pitch. A young person hears roughly 20 Hz up to 20 000 Hz, and the top of that range falls away with age.

The amplitude

提示The size of each vibration — how far the air is shifted every time.

为什么A bigger vibration moves more air, so more energy arrives at your ear. Loudness itself is reported in decibels, and anything above about 85 dB damages hearing if you stay in it.

There are no particles to carry it

提示A vacuum is empty, and this kind of wave needs something to travel through.

为什么Sound moves by particles bumping their neighbours, so it needs a solid, a liquid or a gas. Light needs no such help, which is why you can see a distant star but will never hear one.

The eardrum

提示A tight skin stretched across the inner end of the canal.

为什么Air vibrations shake it, three tiny bones magnify the movement, and the cochlea turns that into nerve signals. Loud noise destroys the delicate hair cells inside, and that damage never heals.

A straight line

提示It is exactly why you cannot see round a corner.

为什么Light keeps going the way it started unless something bends it, which is why shadows have sharp edges and why physicists draw rays with a ruler.

reflection

提示A ray leaving a mirror tilts by as much as the one arriving did.

为什么Both angles are measured from the normal — the line drawn at right angles to the mirror — not from the mirror surface itself. A snooker ball off a cushion obeys the same rule.

The light refracts as it changes speed

提示The rays do not travel through water and air at the same rate.

为什么Light slows on entering water or glass and speeds up again on leaving, and that is what bends its path. Your brain still assumes the rays came straight to you, so it draws the straw in the wrong place.

A spectrum of colours

提示The same band you see arching over a rainy sky.

为什么White light is a mixture, and glass bends each part of it by a slightly different amount, fanning them out from red through to violet. Raindrops do exactly the same job in the sky.

It is absorbed

提示The jumper only sends back its own colour; the rest goes no further.

为什么A coloured surface reflects its own colour and soaks up the others, so a red jumper under pure green light looks black. A filter works the same way, letting one colour through and blocking the rest.

About 340 metres per second

提示Count the gap between a lightning flash and the thunder that follows it.

为什么Sound is around a million times slower than light. Timing an echo lets you measure a distance you cannot reach — but the sound makes the journey twice, so halve your answer.