When you look through a narrow slit, you can see only a thin strip of the world around you. But if you move the slit around rapidly, your eye and brain combine these thin strips to make a single complete picture.
(5 minutes or less)
With a knife, cut a slit in the cap of the mailing tube. The slit should be about 1 inch (2.5 cm) long and 1/8 inch (3 mm) wide. Replace the cap on the end of the tube.
(5 minutes or more)
Close one eye. Put the other eye to the open end of the tube. Cup your hand around the tube to make a cushion between the tube and your eye. Hold the tube so that the slit is vertical.
When the slit is stationary, you can't see much. Keep your head and body still and sweep the far end of the tube back and forth slowly while you look through it. Increase the scanning speed and compare the views. Notice that when you sweep the tube quickly from side to side, you can obtain a rather clear view of your surroundings.
Your eye and brain retain a visual impression for about 1/30th of a second. (The exact time depends on the brightness of the image.) This ability to retain an image is known as persistence of vision. As you swing the tube from side to side, the eye is presented with a succession of narrow, slit-shaped images. When you move the tube fast enough, your brain retains the images long enough to build up a complete image of your surroundings.
Persistence of vision accounts for our failure to notice that a motion picture screen is dark about half the time, and that a television image is just one bright, fast, little dot sweeping the screen. Motion pictures show one new frame every 1/24th of a second. Each frame is shown three times during this period. The eye retains the image of each frame long enough to give us the illusion of smooth motion.
The Viking 1 and 2 landers photographed the surface of Mars by recording narrow-slit images that were transmitted to earth and assembled by computer to make the final surface photographs. As this demonstration shows, your eye and brain can "take a photograph" in the same way.