By Super Awesome Sylvia and her dad, James

Today we’re diving deep for an easy build you can do in an afternoon — your very own cardboard periscope. Let’s go!

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For this reflective build, we’ll need:

• Cardboard. Small milk container, or stiff paperboard
• Ruler, and protractor (optional, but very helpful)
• Scissors
• Tape and/or hot glue
• Pen, pencil or other thing to write with
• Two small handheld mirrors, or anything that’s really reflective and is about the right size.

First, we’ll need a bit of math to get the correct angle to set the mirror, then build around it.

On paperboard/thin cardboard, outline one of your mirrors all the way around, then measure the height (the shorter side). The old makeup mirror we’re using is 5cm tall, so this will be c (the hypotenuse of our right triangle) in our equation.

The formula goes like this: where c is the given height of our mirror, and a will be the length of both of the opposite sides of the right triangle.

First we take our 5cm squared, which is 25, then divide that by 2 and we get 12.5. Now we find the square root of 12 and a half, and we get 3.5… ish. This will be the length of the other sides of our triangle! Draw out extensions of the mirror at that length, then cut around the outside of the whole rectangle. Make two of these, then fold them on the lines, taping the tabs together at a 90 degree angle. Your hypotenuse should come out almost perfect at 45 degrees. Glue or tape on your mirrors, and they’re ready to mount!

Now take one mirror and line up the right triangle with the corner of your cardboard, and make a mark just a bit in front of it. Now turn the mirror lengthwise and make another mark just a bit past it, this will become the front opening. Another mark on it’s end, and then one more mark down the line on it’s side, and you’ve got all four sides of your periscope! Cut off the excess cardboard, then fold or cut the cardboard on the lines to form the body. If you choose to fold, you’ll only have to glue one side, though a strong ruler is highly recommended to prevent bad creases. Now carefully glue in your mirror assemblies on opposite sides, top and bottom, facing outwards, then glue the body shut. Cut away the cardboard in front of each mirror, and you’re done!

If you’ve got a milk carton, it’s a little bit easier. Make sure it’s nice and dry, tape down the carton top, then use a protractor or our previous method to find the 45 degree angle from one side, and mark it. Do the same for the bottom, making sure the two lines are parallel. Using your markings as a guide, cut out the front and back sections in front of the slits (on opposite sides, visualize the path that the light will take) the same height as the lines, and all the way across.

Now carefully slit the lines with your scissors, and slide in your mirrors. With some hot glue and tape, they should stay put at the perfect angle. Decorate with tape to strengthen it, aaaand tada!, your very own super spy periscope! Use it to see over tall fences, around corners, or even check if the coast is clear in your super spy-mobile!

So how does it work? This simple little periscope works on the same basic principals that almost all periscopes and optics share.

First, light enters through the front slot and hits the top mirror. As long as a reflective surface is flat, light will always reflect away at the same angle that it hit, just like a ball bouncing off a wall on a flat surface.

Relative to the mirror at 45 degrees, the light bounces off at the same angle straight down, hitting the other mirror, and canceling out the 45 degree bounce as it exits straight into your eyeballs! It’s simple. Physics Rocks!

On big periscopes like those found in submarines, lenses are used to magnify the image to cover long distances, as crew need to be able to see above the water while they remain hidden and submerged. Maybe once you’ve got some time with your periscope, you can have some submarine adventures of your own.

Try to make your periscope more permanent by replacing the cardboard with wood, or experiment with different mirror angles, or try something crazy with more than two mirrors and longer more curved tubes. Go Nuts!

That’s all we’ve got for this episode, remember to use your imagination, have fun, and get out there and MAKE something!

Check out more episodes of Sylvia’s Mini Maker Show.