Bird in a Cage

1. Cut the same simple shape, such as a bird or a fish, from each of the three colored papers.
2. Glue each shape to its own white board.
3. Draw an eye on each bird or fish with the marking pen.
4. On the fourth white board, if you chose a bird as the shape, draw the outline of a birdcage as in the photo below; if you chose a fish, draw a fishbowl.

   

5. Place the boards in a well-lit area. Bright lighting is a significant factor in making this activity work well.

Assuming you cut out birds, stare at the eye of the red bird for 15 to 20 seconds and then quickly stare at the white board with the birdcage. You should see a bluish-green (cyan) bird in the cage.

Now repeat the process, staring at the green bird. You should see a reddish-blue (magenta) bird in the cage. Finally, stare at the blue bird. You should see a yellow bird in the cage.

If you used a fish shape, try the same procedure with the fish and the fishbowl.

The ghostly images that you see here are called afterimages. An afterimage is an image that stays with you even after you have stopped looking at the object.

The lining at the back of your eye, called the retina, is covered with light-sensitive cells called rods and cones. Rods let you see in dim light, but only in shades of gray. Cones, however, detect color in bright light, and each of the three types of cones is sensitive to a particular range of color.

When you stare at the red bird, the image falls on one region of your retina. The red-sensitive cells in that region adapt to the exposure to red light and reduce their response.

The white board reflects red, blue, and green light to your eyes because white light is made up of all these colors. When you suddenly shift your gaze to the white board with the birdcage, the adapted red-sensitive cells don’t respond to the reflected red light, but the blue-sensitive and green-sensitive cones respond strongly to the reflected blue and green light. As a result, where the red-sensitive cells don’t respond, you see a bluish-green bird. This bluish-green color is called cyan.

When you stare at the green bird, your green-sensitive cones become adapted to the unchanging stimulus. Then, when you look at the white board, your eyes respond only to the reflected red and blue light, and you see a reddish-blue, or magenta, bird.

Similarly, when you stare at a blue object, the blue-sensitive cones adapt, and the reflected red and green light combine to form yellow.

You can design other objects with different colored papers and predict the results. Try a blue banana! For smaller versions, you can use brightly colored stickers (from stationery, card, or gift stores) or drawings on index cards.

One classic variation of this investigation uses an afterimage to make the American flag. Draw a flag, but substitute alternating green and black stripes for the familiar red and white stripes, and black stars on a yellow field for the white stars on a blue field. For simplicity, you can reduce the flag to a few thick stripes and a few large stars. When you stare at the flag and then stare at a blank white background, the flag’s afterimage will appear in the correct colors.

You may also want to experiment by changing your distance from the white board while observing the afterimage. Notice that the perceived size of the image changes, even though the size of the affected region of your retina remains the same. The perceived size of an image depends on both the size of the image on your retina and the perceived distance to the object.

Check out this episode of Build Your Own Exploratorium to see this Snack in action.