Make your own Periscope
|This periscope has an extra feature by which you not only can spy objects over walls and around corners, but also which will show that as you turn one end of the periscope the images turn also. An exercise in light-ray tracing will explain this phenomenon. This toy is easily made of PVC pipes, but you do need to be able to cut 45 degree angle on a 90 degree PVC elbow. An alternative way of making a rotating periscope is given below too.|
One 12" long PVC pipe of 1 1/2 " diameter.
Two 1 1/2 " PVC 90 degree elbows.
Two 2" by 3" mirrors or aluminum Mylar pieces on flat surface such as on a floppy disk.
The only difficult part is to make a 45 degree cut across the outer radius of the 90 degree elbows. You may use a table saw or a hand saw to do this job, but it should be pretty precise. When you tape the mirror onto the cut face of the elbow after the elbow is connected to the pipe, the mirror should be at 45 degrees to the pipe as shown in the picture.
To ease the turning of the elbow you can cut two narrow slots lengthwise about 1 1/2" into the end of the pipe where it fits into the elbow.
Tape the mirrors to the cut face of the elbow.
Alternative construction for Rotating Periscope
One 2" diameter mailing tube of 8" to 12 " long
One sheet of poster board at least 5" by 15 ", preferably black or dark colored
Two plastic mini-cassette boxes of 2" by 3" by 1/2 "
Two 2" by 3" mirrors. If you do not have mirrors you can use Aluminized mylar sheet to make two 2" by 3" mirrors
Double scotch tape
scissors and rulers
scrap fabric materials of 3" by 10 "
Cut along solid lines. Tape pieces together.
1) To make the 45 degree mirror mounts:
Cut three strips 2 1/8" by 4 1/4" using the poster board. (You should be pretty precise, otherwise the mount will fit either too loosely or too tightly.)
Cut one of the above pieces into halves. Now you should have two 2 1/8 by 2 1/8 pieces.
Cut a 45 degree corner off the other piece. To do this, you find the midpoint of the 4 1/4 " side. Draw a line between the midpoint and one corner. You will see a right angle isosceles triangle formed by the two 2-1/8" sides. Cut off this triangle. See figure above.
Repeat the above operation to make another 45 degree cut.
Tape together the four pieces as in the figure, alternating the squares and the 45 degree angle pieces and folding back the second angle piece to join the first square. Orient the 45 degree pieces so that the mirror can be mounted along the 45 degree angle. Now you have a square tube with two 45 degree wings.
Repeat the above and make a second mount.
2) To make the mirror:
If you have two 2" by 3" mirrors, just tape the mirrors to the cassette boxes. Otherwise cut the Al mylar sheet into two 2" by 3" pieces. Using double scotch tape, carefully tape one edge of the mylar piece to the cassette box and pull the mylar taut and tape the other edge to the cassette box to make a wrinkle-free mirror.
3) To mount the mirror:
Tape the cassette box to the 45 degree mount, so the mirror is facing towards the hole. Tape the long side of the cassette case along the 45 degree wings and tape the 2" side to the box to form a hinge-like line. The cassette box sits outside against the edges of wings.
4) To mount the mirror to the mailing tube: Push the mirror mounts over the ends of the mailing tube. Wrap a layer of the scrap material piece to offset any slack between the mirror mount and the tube. The mirror mount should fit the tube snugly but will still rotate easily. Now you can look through your periscope.
To do and see:
1) Hold the periscope so that you can turn the elbow freely.
2) Aim the periscope at something interesting and look through the periscope.
3) Turn the periscope and notice the orientation of the objects.
4) Can you see your friends around the corner?
Why are the images some times up-side-down, sideways or right-side-up ?
This simple periscope has a limited view because it is restricted by the solid angle subtended by the periscope exit end at your eye. The longer the pipe the smaller the angle for viewing.
Periscopes are used in submarines. The top of the periscope has to be above water. The field of view of these instruments, however, is more than just the solid angle subtended at the other end of the pipe, because the real periscope has a much more complicated telescope-like design included in the periscope.
Credit: This Periscope with a Twist was created by Don Rathjen, who taught at the Foothill High School, Pleasanton, CA, and is now with the Teacher Institute of the Exploratorium, San Francisco, CA.
The answer to the ray tracing question is shown below:
In (A) the image will be right-side-up, in (B) the image is up-side-down and in (C) the image is lying on its side.