PINHOLE INVESTIGATIONS

Clamp Light Demonstration
Model of Light as Images

Bob Miller, an artist who worked at the Exploratorium and created many of our exhibits on light and perception, has developed an elegant way of looking at light and images. Most of his ideas and explanations are well described on the Image Walk web site.

Bob developed most of his ideas working with the sun as his "light source." Here we describe an indoor demonstration using an incandescent bulb as a light source. We recommend that you do this demonstration only after people have had a chance to extensively experience and experiment with pinhole images.

Materials

• clear light bulbs with a C-shaped filament
• clamp light with reflector or a small Square Light Source on its side
• aluminum foil-enough to cover the reflector or box opening
• screen or piece of white paper at least 36" x 36"

To Do and Notice

Start with a clear glass light bulb in a clamp light. The bulb should have a clearly distinguishable C-shape filament. Clamp the lamp to the back of a chair, turn off the room lights and shine the light onto a white screen 8 to 12 feet away.

Ask your teachers to look at the screen. What do they see? At first glance it may seem like plain, uninteresting, white light. But Bob Miller sees it as something very different.

Suggest to the teachers that perhaps the light is too bright. Cover the light reflector opening with aluminum foil. Then use a pin to poke a little hole in the center of the foil. A large, C-shape spot of light will appear on the screen. Even though your teachers have experimented with pinhole images, many may be surprised at what they see. Usually someone will realize that they are seeing an image of the filament.

When the group is in agreement that what they are looking at is an image of the filament, poke a second hole to get another image on the screen, then slowly poke more and more holes. Soon some of the images will begin to overlap. Where the images overlap, you will notice that the light is brighter. That's because the light from two images are adding together. Allow the teachers to observe and remark on that phenomenon. That's because the light from two images are adding together. Allow the teachers to observe and remark on that phenomenon.

Continue to poke holes. As the design forming on the screen gets messier and messier, you'll get multiple overlaps. In some areas it will become impossible to distinguish individual images as the multiple overlapping images blend together.

Finally, when there is more light than darkness on the screen, use the pin to just rip away the remaining foil. On the screen you will again see an expanse of diffuse white light. But you won't think of it as just plain white light anymore. What you have is a confusion of filament images.

What's Going On

"Plain" light is not plain light, it is information in the form of images. We normally can't see those images because there is too much information, too many images tangled and overlapping, creating a grand confusion. What the pinhole does for us is isolate a single image by blocking out other images.

This model of light being composed of a multitude of images is a powerful means of thinking about the world.

For instance, we can be apply this model to outdoor sunlight. When we go outdoors on a sunny day we see bright, uniform sunlight on the ground. But under a tree, we may see round patches of light. The tree leaves block out most of the sun images. But the spaces between the leaves can isolate single images of the sun and allow them to pass through, creating a sun image on the ground. We can then understand that the diffuse sunlight which we initially saw is actually made up of many overlapping sun images.