How to Build a Simple Periscope from a Milk Carton

Learn how to make a working periscope using a milk carton and two mirrors for a fun science experiment with your kids.

1. Gather Your Materials. You'll need a clean, empty half-gallon milk carton, two small mirrors (about 3-4 inches wide), scissors or a craft knife, tape or glue, a ruler, and a pencil. Small makeup mirrors or mirror tiles from a craft store work perfectly. If you don't have mirrors, you can try using reflective materials like polished metal or even old CDs, though mirrors work best.
2. Prepare the Milk Carton. Rinse the milk carton thoroughly and let it dry completely. Keep the carton sealed at the top. Using your ruler and pencil, mark a square window about 2 inches wide on one side of the carton, roughly 2 inches from the bottom. Then mark another identical window on the opposite side of the carton, but this time about 2 inches from the top. The windows should be on opposite sides and at opposite ends of the carton.
3. Cut the Viewing Windows. Carefully cut out both square windows using scissors or a craft knife. Adults should handle the cutting for younger children. Make sure the edges are smooth and the windows are large enough to see through clearly. Test by looking through the carton from one window to the other to make sure you have a clear line of sight.
4. Position the Mirrors. This is the tricky part that makes the periscope work. Place the first mirror inside the carton at a 45-degree angle near the bottom window, with the mirror surface facing up and toward the top of the carton. The mirror should reflect light coming down from above and send it toward the bottom window. Place the second mirror near the top window, also at a 45-degree angle, but with the mirror surface facing down toward the bottom of the carton. This mirror catches light from the top window and reflects it down through the carton.
5. Secure the Mirrors. Once you have the mirrors positioned correctly, use tape or glue to hold them firmly in place. Make sure they stay at that important 45-degree angle. You might need to use small pieces of cardboard or folded paper to prop the mirrors at the right angle before taping them down. Test your periscope by looking through the bottom window while pointing the top window at something interesting.
6. Test and Adjust Your Periscope. Look through the bottom window and see if you can clearly see what's visible through the top window. If the image looks fuzzy or you can't see anything, adjust the angle of your mirrors slightly. The mirrors need to be parallel to each other and both at 45-degree angles for the periscope to work properly. Once everything looks clear, secure any loose mirrors with additional tape.