How a periscope uses two mirrors
Physics · Key Stage 2 · The School
Seeing over an obstacle
A PERISCOPE lets you see over an obstacle you cannot see over directly. It needs nothing but two flat mirrors and a straight tube. Submarines use large ones, to see the surface while staying underwater. A simple one can be built from a cardboard tube and two small mirrors.
Two mirrors, angled to match
Both mirrors are angled at exactly 45 degrees, facing each other. One sits at the top of the tube, one at the bottom. Light from above hits the top mirror and reflects DOWN the tube. It then hits the bottom mirror and reflects again, this time OUT towards your eye. Two reflections happen. The image has travelled round a right angle each time, without ever leaving a straight path of mirrors.
Why the angle has to be exact
The rule of reflection says the angle light arrives at equals the angle it leaves at. A mirror tilted at exactly 45 degrees turns a beam through exactly 90 degrees. Tilt either mirror even slightly wrong, though, and the image no longer lines up with your eye at the other end. The two mirrors have to agree with each other, not just each be roughly angled.
A ray of light leaves an object above the top of the periscope. It travels straight down towards the top mirror. It strikes the top mirror at 45 degrees, and reflects. It now travels straight along the tube, horizontally, towards the bottom mirror. It strikes the bottom mirror at 45 degrees and reflects again, now travelling straight towards your eye. The ray has changed direction exactly twice, by 90 degrees each time. It never once bent or curved. Light only ever travels in straight lines, and every direction change happened at a mirror.