Seeing colour
Afterimages
Stare at a colour, look away, and you see its opposite floating in front of you. Tired receptors are the cause.
Presented by ClearTrust Free for designers, students and the curious.
An afterimage is a picture that stays in your vision after you stop looking at something. Stare at a red shape for half a minute and then look at a white wall, and a pale blue-green copy appears. The colour is roughly the opposite of the one you stared at. It fades within seconds and moves with your eyes, because it is in your visual system and not on the wall.
Why the colour flips
Receptors and nerve cells respond less the longer they are kept busy. This is called adaptation. Stare at red and the cells that signal red grow weaker. Look at white, which would normally stimulate everything equally, and the weakened red signal loses out to its opponent. The balance tips towards green and blue, so that is what you see. The same thing happens with lightness. A bright window leaves a dark patch. This kind, in the opposite colour, is called a negative afterimage, by analogy with a photographic negative.
Positive afterimages
There is a second, briefer kind. Glance at a camera flash or a bare bulb and for a moment you see a bright spot in the same colour as the source. This is a positive afterimage, caused by receptors that carry on firing after the light has gone. It usually gives way to a negative one within a second or two. The persistence is short, but it is part of the reason a row of film frames or a spinning firework looks like continuous movement instead of separate flashes.
Where you meet them
Surgeons' gowns and theatre drapes are green or blue-green partly for this reason. After looking at red tissue, a surgeon who glances at white cloth would see distracting green ghosts. On green cloth they do not show. Painters and designers meet the effect whenever strong colours sit together: the afterimage of one tints the next thing looked at, which is one cause of the vibrating edge between bright opposites. Afterimages are normal. One that lasts for minutes, or appears with no bright source, is worth mentioning to an eye doctor.
How to do it
- Draw or find a solid red circle about the size of a coin on a white page.
- Stare at its centre for thirty seconds without moving your eyes.
- Look at a blank white area and blink once or twice.
- A pale blue-green circle appears and fades. Try again with a yellow shape and note the violet-blue result.
Sources and further reading
- Afterimage on Wikipedia
- Neural Locus of Color Afterimages, Current Biology (2012)
- Webexhibits, Color Vision and Art: Afterimages
Original text by the Rang desk, written with AI assistance and checked against the sources above. Meanings of colour vary from place to place and person to person. If we have something wrong, tell us.