Your eye and brain can hold on to a series of images long enough to form a single complete picture.
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.
Cardboard mailing tube about 3 inches (8 cm) in diameter and 2 to 3 feet (60 to 90 cm) long
A cap for the tube or black paper and tape
Ruler and pencil
If your mailing tube has a cap, use the pencil and ruler to mark a rectangle that is 1 inch (2.5 cm) long and 1/8 inch (3 mm) wide. Use your utility knife to cut out a slit the size of the rectangle. Replace the cap on the end of the tube.
Alternatively, tape two small pieces of black paper to one end of the tube so that there is a 1/8 inch (3 mm) slit between them.
To Do and Notice
Close one eye. Put your open eye to the open end of the tube. Cup your hand around the tube to make a cushion between the tube and your eye and prevent light from leaking in. 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. (Make sure the area around you is clear.) Increase the scanning speed and compare the views. Notice that when you sweep the tube quickly from side to side, you can see a remarkably clear view of your surroundings.
What's Going On?
Your eye and brain retain a visual impression for about 1/30 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 traditional television image is just one bright, fast, little dot sweeping the screen. Motion pictures show one new frame every 1/24 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 stitch together a picture in the same way.