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Principles of a homemade periscope

Build a homemade periscope

Parents have eyes in the back of their heads – we all know that.  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 provide an explanation of this natural phenomena.  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…

  1. Cut off the angled top of the milk cartons so each carton has a squared off, open end.
  2. Where to cut the holes in the milk carton for your homemade periscopeOn 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.
  3. 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.
  4. 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.
  5. 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.
  6. 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 mom and dad sneak up on you by crawling on the ceiling, this doesn’t do us much good.
  7. Repeat steps 1-6 with the second milk carton.  This time, if you pause on step 5 to admire yourself in the mirror, you probably have a problem.
  8. 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.
  9. 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 of an object, you can understand why this “periscope” uses bouncing light waves to let us see around a corner.

Principles of a homemade periscope

Here are some 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.

Parent/Teacher/Advanced Notes

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

Experiment Supplies

Supplies: Mirror, Milk carton, Glue


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