Build a homemade periscope using a cardboard milk carton

We all know parents have eyes in the back of their heads. They always know what you’ve done, where you did it, and sometimes even why you did it. We’ll never fully understand how they do this; even science cannot explain this natural phenomenon. But this experiment can at least put you on somewhat equal ground as we create a gadget that will literally let you see around corners…

Cut off the angled top of the milk cartons so each carton has a squared off, open end.
On the closed end of the carton, cut a small square hole on one of the sides. Leave about ¼ inch of carton on each side of the hole. This hole will be the part that you look through.
Lay the carton down on its side so that the hole you just cut is on the left-hand side and facing you. Above this hole, on the top of the carton, draw a line beginning at the top left-most corner closest to you, that angles back towards the back of the carton. The line should be a 45-degree angle and is marking where we will insert the mirror. The mirror will be placed inside the carton at this angle so when peering through the hole that you cut, you will be looking directly at the angled mirror. See the sketch for a diagram of how the line should be drawn.
Use a knife to cut along the line. Don’t cut all the way to the edges – only make the line as long as the mirror.
Slide the mirror through the slot so the mirrored surface is facing the hole you cut. It is OK to admire yourself in the mirror before sliding it through the slot.
Hold the carton up and look through the hole. With the mirror angled, you should see a reflection of the ceiling. Although this is really cool, unless your parents sneak up on you by crawling on the ceiling, this doesn’t do us much good.
Repeat steps 1-6 with the second milk carton. This time, if you again pause to admire yourself in the mirror, you probably have a problem.
Squeeze the open end of one of the cartons just a bit so that you can slide it inside the open end of the other carton. One hole and mirror should face you while the other hole and mirror should face the opposite direction.
Look through one of the holes. If the mirrors are lined up right, you should be able to see the reflection that passes through one hole by looking into the other hole.

Given that what we see with our eyes is simply light waves bouncing off an object, you can understand why this “periscope” uses bouncing light waves to let us see around a corner.

How a periscope works

A periscope is a device that allows one to see objects that are not in the direct line of sight. It consists of two mirrors or prisms, which reflect light at an angle. The mirrors or prisms are arranged parallel to each other and at a specific distance apart.

Periscope diagram using mirrors or prisms — Principle of the periscope. The periscope on the left uses mirrors at locations labeled “a” whereas the right uses prisms at “b” locations. The observer is “c”.

The periscope’s operation is based on the reflection of light. Light travels in a straight line, and when it encounters a mirror or prism, it bounces back at the same angle at which it hit the surface. The two mirrors or prisms in a periscope are arranged to reflect the light at a 90-degree angle.

When light enters the periscope, it first hits the top mirror or prism, which reflects it downward at a 90-degree angle. The light then hits the bottom mirror or prism, which reflects it again at a 90-degree angle, back up to the viewer’s eye. The image that the viewer sees is the result of the reflection of light by the two mirrors or prisms.

Periscopes are useful tools for a variety of applications. They are commonly used in submarines, tanks, and other military vehicles to allow the viewer to see objects above the waterline while remaining hidden underwater or behind obstacles. Periscopes are also used in many other applications, such as in surveillance, photography, and astronomy.

Interesting facts about periscopes

Periscope comes from two Greek words – peri meaning “around” and scopus meaning “to look”.
Light reflects away from the mirror at the exact same angle as it hits it. With a 45-degree angle, the light will bounce 45 degrees to the other mirror.
The further apart the mirrors are the smaller the resulting image. Periscopes on submarines work on this same principle but use magnifying glasses in between the mirrors to magnify the image.
In the 1950s, periscopes were used in some American schools to allow students to see over the heads of taller students in crowded classrooms.
Periscopes have also been used in the entertainment industry, for example, to capture shots in movies or TV shows that would be difficult or dangerous to film otherwise.
The first patent for a periscope was filed in 1854 by Hippolyte Marié-Davy.

Build a homemade periscope experiment advance notes

You can conduct a similar experiment using PVC pipe. Use the correct angles and glue the pipe such that it forms an S-shaped pipe. Glue the mirrors inside the angled pipe pieces (before you glue all the pieces together).