Waves, light and sound

Energy

HintNot the water itself — a duck sitting on a ripple bobs but stays put.

The whyA 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

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

The whyEvery 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

HintTighten a guitar string and listen to what the note does.

The whyFrequency 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

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

The whyA 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

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

The whySound 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

HintA tight skin stretched across the inner end of the canal.

The whyAir 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

HintIt is exactly why you cannot see round a corner.

The whyLight 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

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

The whyBoth 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

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

The whyLight 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

HintThe same band you see arching over a rainy sky.

The whyWhite 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

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

The whyA 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

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

The whySound 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.