If the only mirrors you can find are larger than that, you can use half-gallon milk cartons instead. When you are making a periscope, it's important to make sure that your mirror is positioned at a 45-degree angle. If you use a wider milk carton or some other box, just measure how wide your box is. Then measure that same distance up the side of the box and make a mark. The line between your mark and the opposite corner of the box will be at 45 degrees.
Light always reflects away from a mirror at the same angle that it hits the mirror.
In your periscope, light hits the top mirror at a 45-degree angle and reflects away at the same angle, which bounces it down to the bottom mirror. That reflected light hits the second mirror at a 45-degree angle and reflects away at the same angle, right into your eye. You can make your peri- scope longer, but the longer the tube is, the smaller the image you'll see. Periscopes in tanks and submarines have magnifying lenses between the mirrors to make the reflected image bigger. This and dozens of other cool activities are included in the Exploratorium's Science Explorer books, available for purchase from our online store .