This is an example of an optical illusion:
- Stare at the eye of the red bird while you slowly count to 20, then immediately look at one spot in the empty birdcage. The faint, ghostly image of a blue-green bird should appear in the cage.
- Try the same thing with the other bird, and a faint bird should appear.
The ghostly birds are called afterimages. An afterimage is an image that stays with you even after you have stopped looking at something. As you stare at the red bird, light-sensitive cells at the back of your eyes become less responsive to red light. When you shift your gaze to the birdcage, your visual system “subtracts” red light from the white light that’s being reflected from the white background. White light minus red light is blue-green light. That’s why the afterimage of the bird is blue-green. A similar thing happens when you stare at the other bird.
