Waves, light and sound, Year 9

White light

提示It is what the Sun and a classroom projector lamp give out.

为什么It looks like the absence of colour, but it is every colour arriving together. A prism does not add anything to it; it only spreads out what was already there, and a second prism can bring the colours back together again.

Each colour is bent by a different amount

提示They all change direction at the glass, but not equally.

为什么Light changes direction when it enters and leaves glass. Each colour changes direction slightly differently, so colours that went in together come out heading in slightly different directions and fan out into a band.

Violet

提示It sits at the opposite end of the band from red.

为什么Red is turned the least and the colour at the far end of the band the most, with the others in order between them. That is what fixes the order of the spectrum, and it is the same order every time, in a prism or in a rainbow.

yellow; blue

提示Many people learn the order from the name Roy G. Biv.

为什么The order never changes, because it follows how much each colour is bent by the glass, from least to most. There are no sharp edges in a real spectrum: each colour shades gradually into the next.

Black

提示Ask what the book does with red, and then what is left to reach your eye.

为什么A blue surface reflects blue light and absorbs the other colours. Under pure red light there is no blue to reflect, and the red is absorbed, so almost no light comes back from the book at all.

A colour filter

提示A coffee machine uses a paper one to hold back the grounds.

为什么Shine white light at a green one and only green comes out the other side; the rest of the spectrum is absorbed inside it. Stage lights and traffic lights have often been made this way, with a white lamp behind coloured glass.

No light

提示Work out what survives the first sheet, then ask what the second sheet does to that.

为什么The red filter lets only red through. The green filter absorbs everything except green, and there is no green left in the beam, so nothing gets past it. Each filter can only take colours away.

reflects

提示It is what a mirror does to all the light that reaches it.

为什么Daylight contains every colour. The skin of the apple takes in most of them and sends back mainly red, and that returning light is what your eye receives. The colour you see is whatever is left after the absorbing.

It reflects every colour, and only red is arriving

提示Think about what a white surface does in daylight, then about what this lamp is sending.

为什么A white surface reflects all the colours that land on it, which is why it looks white in daylight. Give it only one colour and that one colour is all it can send back. A white shirt takes on the colour of whatever light is shone on it.

510

提示The sound has made the trip twice by the time you hear it.

为什么In 3 s the sound travels 340 × 3 = 1020 m. That is the distance to the cliff and back again, so the cliff is half of it away. Forgetting to halve is the commonest slip in any echo question.

Light travels far faster than sound

提示Both set off together from one place, so compare how quickly each makes the trip.

为什么The flash and the bang are made at the same instant. The flash reaches you almost immediately, while the thunder crosses the air at about 340 metres every second, so it lags further behind the further away the storm is.

An echo

提示You get one by shouting in a tunnel or towards a cliff, and then waiting.

为什么Hard, flat surfaces such as cliffs and walls reflect sound well; soft ones such as curtains and carpet absorb it. You hear the reflection as separate from the original only when the surface is far enough away for the delay to be noticeable.

About 2 km

提示This sound made a one-way trip, so use the whole time.

为什么Distance = speed × time, so 340 × 6 = 2040 m, which is just over two kilometres. The light arrives so quickly that its travel time can be ignored. Unlike an echo, the thunder has travelled the distance once, so nothing is halved.