Making periscopes is easy, and enables students to explore how light is reflected by mirrors. By designing different types of device, your students will develop an appreciation of how mirrors work.

This experiment uses mirrors, but most periscopes used in submarines are complex optical devices, many of them making use of prisms and lenses as well as mirrors.

To make a basic periscope you need a tall cardboard tube. You can use almost anything, but a square-shaped tube tends to be easier to use. Two fruit-juice cartons or milk cartons can work well, or you could make a square tube from cardboard.

You also need two flat mirrors, preferably the same size. Differently sized mirrors will work, but make the build slightly more difficult.

Place your first carton on the table, and remove the top. Cut out a viewing window at the front of the carton near the bottom, making sure it is a similar size to the mirrors you are using.

Place the mirror at an angle of 45 degrees to the window, facing upwards.

Measure the width of the sides of the box. To position the mirror, mark a line across the sides of the box this distance from the bottom, so that you have marked off a square.

Draw a diagonal line across the square on each side of the box, making sure that each line starts from the bottom corner closest to the aperture.

Showing pupils a diagram will make this much clearer.

Use these diagonal lines to cut slots in the box. Push the mirror through the slots, and fix it in place with sticky tape. Repeat with the second carton. Turn the second carton upside down, so that the mirror is now pointing downwards. Turn this second carton through 180 degrees, so that its aperture is now facing towards the back. Slightly crumple the open end of this carton so that it slots into the first carton. The two mirrors should be parallel, the reflecting surfaces facing each other.

Now look through the bottom aperture, and you should see the reflection of anything the top aperture is pointing at.