Using a slide projector as a light source, you can duplicate many of the effects you observed on the Image Walk and extend your investigation of light and images to include lenses, a tool that you can use to manipulate images.
On the Image Walk, you used a filter and a screen to isolate the image of a light source, the sun. When you show slides, you use the same basic ingredients: a light source, a filter, and a screen. Light from the projector's light bulb passes through the slide, and the three informationcarrying characteristics of the light - the frequency or color, the direction, and the intensity - change. As a result, the light carries an image of the slide. Using your slide projector, a filter, and a screen, you can isolate both the image of the slide projector's light source and the image of the slide through which the light is shining.
The instructions here are intended to help you get started in a more advanced investigation of the properties of images. The light shining from the slide projector - like the sunlight in your playground - is more interesting than you may think. Try a few simple experiments with your slide projector. You will have to play around to see some of the effects we have found and to find the methods that work best with your equipment. Don't be afraid to experiment to find the best conditions. At the Exploratorium, we have found some of the most interesting effects when we were just "tinkering around" with the slide projector and pinholes. Science involves a great deal of "tinkering around," a process integral to research and development.
You can use these activities as a class demonstration or as learning stations.
Slide projector with removable lens
35mm slide of recognizable object
Large sheet of white posterboard (at least 23" by 29") for a screen
A few sheets of heavy card stock (like index cards, NOT cardboard)
Thumbtack or pushpin
During these activities, you will see the image of the projector's light source: the filament of a light bulb. Before you start experimenting, read "Inside Your Slide Projector" near the bottom of this page and take a look at the bulb in your projector, so that you will recognize the image of the light source.
Most slide projectors have an adjustable lens that moves in and out when you focus the image of a slide on a screen. For some of these activities, you must remove this lens from the projector. In the projectors we examined, we used the focus adjustment to move the lens as far out as possible, then pulled the lens from the projector with a gentle tug. Examine your slide projector and figure out how to remove the lens.
You will also need a 35 mm slide. Since the images you create using this slide will be somewhat blurred, the photo should show a building or a scene that is familiar, so that students will recognize a blurred image. Experiment with a few slides to find the best one. We use the slide of the white geometric solids you see to the right. You will also need a large sheet of stiff white posterboard (measuring about 23" by 29") to use as a portable screen, a few sheets of non-corrugated cardboard (measuring about 3" by 5") to cover the projector's lens, and a thumbtack or pushpin.
Set up your slide projector and insert a slide. Remember that to make the image appear on the screen right-side-up, you must insert the slide upside-down. Focus the image of the slide on a white wall or a screen placed about 7 feet from the projector. What do you suppose would happen if you blocked some of the light from the projector? Try it and see.
Hold your hand or a piece of cardboard up against the lens, blocking half the light from the projector. The image on the screen dims, but remains intact. Try blocking even more of the light. As long as a little light gets through, you will see the complete image of the slide. Each little bit of the light passing through the lens carries all the information needed to make an image.
Remove the adjustable lens from the projector. The image of the slide on the wall will become a blur of colored light. Use a thumbtack to punch one hole in a sheet of cardboard, then hold the cardboard over the opening from which you removed the lens. Let light from the projector shine through the hole onto your posterboard screen, held about 8 inches away. You will see a clear image of the slide on the posterboard screen. Your pinhole isolates one recognizable image.
Punch one or two more holes in the cardboard. Group the holes close enough to each other so that you can position the card with all the holes over the lens opening. Within this constraint, place the holes as far apart as possible. Hold the cardboard so that the light shines through the holes onto the screen. You will have as many images on your posterboard screen as you have holes in the cardboard. Move the screen farther from the projector. The images will grow larger, but remain separate.
Replace the projector's original lens and let the light shine through the holes again. If you hold your posterboard screen about a foot from the projector, you will see one image from each hole in the cardboard. The images may overlap, but each one will be complete. As you move the screen back slowly, the images will overlap more and more. When your screen reaches the position where the image was originally focused, the images will merge to form a single image.
Remove the slide. Now you can reveal the image of the light source in the same ways that you revealed the image of the slide. Hold your pinhole about a foot away from the projector so that the light shines through the hole onto the screen 7 feet away. The light shining through the hole will make an image of the light source. Depending on the shape of the filament in the lamp, you will see a different shape on your screen. Four different filaments (2 with circular reflectors) can be seen in the pictures to the right. Use a thumbtack or pushpin to punch an array of holes in a sheet of cardboard. Space the holes about 1/2 inch apart. Hold the cardboard about one foot from the projector so that the light shines through the holes. Notice that each hole shows a slightly different view of the bulb.
Lenses and Images
Most of the effects that you observed in this activity are very similar to the effects you observed on the Image Walk. In both you used filters to isolate an image. In both you showed that each bit of light carries an image of the light source.
When you experimented with the slide projector, you used one tool that you may not have used on the Image Walk: a lens. When you have no lens in your slide projector, the light transmitted by the slide radiates in all directions. Each part of the light carries the image of the slide, but the multitude of images overlap in a blur of light. By letting a tiny bit of light shine through a hole, you can isolate one image.
Lenses are designed to bend light. A convex (curved out) lens, like the lens in your projector, bends light rays together. When a lens bends light, it also moves the image that the light carries. By bending light rays together, a convex lens stacks many images in the same place. Because the lens overlaps many images, the picture on the screen is much brighter than the image you would see with a single pinhole.
In Activity 2, you removed the lens from the projector and let light shine through two pinholes, isolating two images of the slide. Then you used the projector's lens to stack the images from several pinholes. When you held the posterboard screen near the projector, you could see one image from each hole. As you pulled the screen back, the images moved closer and closer together, until they overlapped completely. The lens bent the light to stack these images. When all the images overlapped completely, the slide was in focus.
You will find convex lenses in telescopes, microscopes, magnifying glasses, cameras, slide and movie projectors, and other instruments that use light to create an image.
Inside Your Slide Projector
Though each slide projector is a little different all projectors have a few basic components. Open your projector as if you were going to change the light bulb and look for these parts:
1) The light bulb and reflector.
Every projector has some light source. The bulb produces light; the reflector located just behind the bulb reflects light in the direction that you want it to go. In some projectors the reflector and the bulb are one unit. In all projectors, the bulb-and-reflector combination is designed to provide a bright light. You may have to remove the bulb to take a close look at the filament, the coiled wire, which glows when the bulb is on. Avoid touching the bulb with your fingers; the oil from your skin can reduce the life of the bulb. If you do touch it, don't worry, just clean the bulb with a facial tissue and some rubbing alcohol.
2) Heat-absorbing glass.
Between the light bulb and the slide you may find a thick piece of greenish glass. This glass prevents much of the heat produced by the bulb from reaching the slide.
3) One or two condensing lenses.
The projector is designed so that as much light as possible passes through the slide. The condensing lens (or lenses) catches the light from the light bulb and reflector and bends it so that it passes through the slide.
4) Adjustable lens.
This is the lens that you adjust to focus the slide. It catches the light that has passed through the slide and bends it to form an image of the slide on the screen.