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. Its cause is not well known. It likely comes from cells in the retina that 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. A very bright source can cause flash blindness, a stronger effect in which the light bleaches the pigment in the retina².

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 colour channel that responds to the colour you stared at adapts and sends a weaker signal. When you then look at a white surface, the brain reads the weak signal as its opposite colour. 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 1881³.

How common it is

Afterimages are not tracked by prevalence studies. They come from ordinary wiring in the retina and visual cortex that everyone with working photoreceptors has¹. Positive afterimages follow any bright flash, and negative afterimages follow any long stare at a strong colour, so almost anyone can produce one on demand. The effect needs no special sensitivity, only enough contrast and enough time looking at one spot.

Try it

The face below is drawn in opposite colours. Every colour in it is the exact inverse of a normal one.

1. Stare at the dark dot between the eyes for 30 seconds. Keep your eyes still.
2. Then look at a plain white wall, or close your eyes.
3. The same face appears in normal colours. The blue-green skin turns warm, the pale hair turns dark, and the green lips turn red. It fades in a few seconds.
4. Blink slowly. The afterimage often comes back briefly each time.
5. Look at a wall further away. The face looks larger there. That is Emmert’s law.

Is it a problem?

No. Afterimages are a normal side effect of how the retina and colour vision work, not a sign of eye damage¹. Flash blindness from a very bright flash usually fades within seconds to minutes, as the retinal pigment returns to normal². Light that is bright enough, such as a nuclear explosion, can cause permanent damage².

Mention it to a doctor if an afterimage-like patch:

• lingers for many minutes or hours,
• keeps appearing with no bright trigger,
• comes with other new visual changes.

That pattern is not ordinary afterimages and is worth a look.

History and culture

Negative afterimages are used in classroom demonstrations of colour vision. One example 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.