Afterimages

How it works

There are two kinds of afterimage. They come from two different mechanisms¹.

A positive afterimage keeps the same colours as the original. It lasts a fraction of a second. It appears because the cells in the retina keep firing for a short time after the light stops¹. You see it when a camera flash goes off or when you glance at a bright lamp. Flash blindness from a very bright source is an extreme version of the same thing².

A negative afterimage shows the opposite colours. Red becomes cyan, yellow becomes blue, and bright becomes dark. It appears after you stare at something for many seconds. The cone cells that respond to the colour you stared at become less sensitive. When you then look at a white surface, those cones respond weakly and the other cones respond normally. The result is the complementary colour¹. This is called adaptation.

The afterimage is fixed on the retina, so it moves when your eyes move. It also appears to change size. Look at a nearby wall and the afterimage looks small. Look at a distant wall and the same afterimage looks large. This is called Emmert’s law, and it was described in 1881¹.

Try it

1. Find a bright red shape on a white background. A red circle on a screen works.
2. Stare at the middle of it for 30 seconds without moving your eyes.
3. Look at a plain white wall or a blank page.
4. A cyan or blue-green circle appears where the red one was. It fades in a few seconds.
5. Blink slowly. The afterimage often comes back briefly each time.
6. Repeat with a blue shape. The afterimage is yellow.

History and culture

Negative afterimages are used in classroom demonstrations of colour vision. The most famous version is an American flag drawn in green, black and yellow. Stare at it, look at white, and you see the real colours¹. Some artists have used the effect on purpose in paintings and posters.