Light travels in a straight line

On the bus, Keshav notices that light seems to travel in a straight line. Can we test that?

Activity 11.1: make a hole in the inner tray of three matchboxes, at exactly the same spot. Stand them in a row so the three holes are at the same height and in line. Put a torch on one side, with its bulb level with the holes, and a cardboard screen on the other. You get a bright spot on the screen.

Now push one matchbox slightly to the side, or up, or down. The spot disappears. Light gets through only when all three holes are in a straight line.

Activity 11.2 (with a lighted candle, under adult supervision): look at the flame through a long, bendy pipe held straight. You can see it. Now bend the pipe and look again. You cannot.

Both tests say the same thing: light travels in a straight line.

You can even see the straight path. Shine a laser beam through a beaker of water with a drop of milk stirred in; the milk makes the beam show up, and it goes perfectly straight. Lasers need care: use one only with a teacher, only a low-power pointer, and never shine it into anyone's eyes — it can badly damage them. The same goes for looking straight at the Sun or a very bright lamp.

The book adds, honestly, that light can bend round corners slightly in some cases — something you will learn about in higher grades.