Colour illusions

After-images: Jumping fish

Click on the adjacent interactive button. Concentrate on the eye of the fish for 30 seconds. Then immediately look at the spot in the fish bowl.

You should see an orange image of the fish in the bowl. This is called an after-image. Such after-images can help us understand how we see in colour. By staring at the blue fish for an extended period an area of your retina becomes less sensitive to blue. When you look at the white background of the bowl the colour which you see is white minus blue, which is orange.