Poking Fun at Light Rays

Build your pinhole viewer:

1. Cut the wax paper to a size slightly larger than the diameter of the tube. Do the same with the aluminum foil.
2. Cover one end of the tube with the cut piece of aluminum foil, folding the ends over tightly and securing the foil in place with a rubber band.
3. Cover the other end of the tube with the cut piece of wax paper, also folding the ends over tightly and securing in place with a rubber band. Make sure the surface of the wax paper is as smooth and wrinkle free as possible; this will be your viewing screen.
   
4. Optional: for better viewing, you may want to add a shade to your screen (wax paper). Roll the black construction paper around the tube so the wax-paper end is enclosed, leaving the foil end exposed by a few inches (10 cm). Secure the construction paper in place with rubber bands.

Put together your light source:

1. Screw the red, green and blue bulbs into the light sockets.
2. Plug one, two, or three bulbs into the power strip and turn it on.
   

Set up your light source several feet (1 m) away. Make the room as dark as possible, then start by turning on only one of the colored bulbs.

Poke a small hole in the center of the foil with the pushpin. Orient your viewing device so the foil side is facing the light and the wax paper is facing you. Hold it up to the light, about 8–10 in (20 cm) from your eyes, and look at the wax paper as if it were a small television screen. What do you see?

Turn on a second light bulb, then look through the viewer again. Have your partner move and rotate the lights in different directions. Then ask your partner to block one light at a time and move his or her hand (or another object) in front of the lights, from right to left, left to right, top to bottom, etc. Is what you see on your screen upside down or reversed from left to right?

Hold your viewer up to another light source; for example, look outside through a window or door. Try other experiments and see what you observe.

Light rays travel in straight lines and in all directions from a light source. However, your pinhole device limits the rays that can reach the wax-paper screen. The aluminum foil blocks all the light except what passes through the pinhole.

The geometry of how those rays pass through the pinhole is what causes the image to form upside down and inverted on the screen. Light rays from the top of an object pass through the pinhole and are projected onto the bottom of the viewer’s screen. Likewise, light rays emanating from the bottom of the object pass through the pinhole and are projected onto the top of the screen. The same is true for rays of light traveling from the right and left sides of the object, they are projected onto the opposite sides of the screen. These inverted rays are what form the inverted image (click to enlarge diagram below).

A pinhole viewer (also known as a camera obscura) is a good starting point for many scientific inquiries and engineering challenges. For example, how can you improve or enhance the pinhole viewer design? Can you make your image larger? Brighter? Sharper? How will changing the materials you use affect the viewing experience? Try looking at other bright objects.

What other discoveries can you make with your viewer? Did you encounter problems while making it? What new questions do you have after looking through the viewer? How can you pursue your questions? What data will you need?

Experiment, have fun, and try our other related Snacks!