You can’t see around corners… or can you? In this activity, carefully placed mirrors change the path light takes twice, making it possible to look over walls, around corners and even behind you – all without being seen.
What’s the science?
A periscope allows us to see things that are otherwise out of sight. We see an object when light coming from it enters our eyes. The light may have been made by the object itself, or it may have simply bounced off it. Either way, the light travels in straight lines – so normally you have to look straight at something to see it. But when light hits a mirror, it bounces off – at the same angle it hits. Light travelling horizontally meets the first mirror in the periscope at 45 degrees, and bounces off at 45 degrees, making 90 degrees altogether – so it ends up travelling vertically downwards.
Mirrors allow you to see things that aren’t in your direct field of view. A rear-view mirror in a car, for example, reflects light coming from behind the driver into their eyes so that they can see cars and other objects behind them. And at the hairdresser’s a combination of two mirrors allows you to see the back of your head.
Johannes Gutenberg, who is better known for the printing press, invented the first periscope in the 1430s, so that pilgrims could see over people’s heads at religious festivals.