Waves
Light, shadows and reflection
Tell luminous objects from non-luminous, use the straight-line rule to explain shadows, and give the four properties of an image in a plane mirror.
Look through a straight hollow pipe at a candle flame and you see it. Bend the pipe and the flame disappears, though nothing has moved but the pipe. That one observation carries most of this chapter: light travels in straight lines. Shadows, mirrors and the pinhole camera are consequences of it.
Who gives out light, and who passes it on
A luminous object emits light of its own: the Sun, the stars, lightning, a firefly, a lit lamp. A non-luminous object does not — the Moon shines because sunlight falls on it and bounces off, and so does a mirror. Materials differ too: light passes almost completely through a transparent one, partially through a translucent one, and does not get through an opaque one.
Have a play
A torch, an opaque object and a screen — the three parts of a shadow set-up.
Tap any part of the picture.
Worked example
Three matchboxes each have a hole pierced at the same height. Lined up in a row with a torch at one end, a bright spot appears on a screen at the other. One box is nudged sideways and the spot vanishes. What does this show?
Note what stayed the same. The torch is still on, the screen has not moved, and the holes are still the same size.
The nudge changed one thing — whether the three holes lie on one straight line — so that is what the vanished spot is about.
Try it together
Stand in front of a plane mirror and work out four things about what you see.
Each step takes a word or a short phrase.
1.You raise your left arm. Which arm does the image appear to raise?
Have a go
Have a go on your own. Two plane mirrors fixed in a Z-shaped box let a submarine's crew see above the water. What is that device called?
Ready to practise?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.