Stare hard at a colored shape, then look away — what shows up?
After you watchStare hard at a colored shape, then look away — what shows up?
The short answer
Stare at a bold color for a while and your eye's sensors for that color get tired and fire less. When you then look at a plain white wall, the sensors that stayed fresh out-vote the tired ones, so you see a faint ghost of the shape in the opposite color — stare at red and the ghost is cyan.
Try this next
- What if you stare at blue instead of red? Tap the blue chip and stare again — predict the ghost color before it appears, then check it's the opposite of blue.
- Does a longer stare make a stronger ghost? Run the stare once quickly in your head, then really hold your eyes still for the full timer — notice how much bolder and longer-lasting the ghost gets.
The whole story
How it works
The back of your eye has three kinds of color sensors, roughly tuned to red, green, and blue. Looking at a steady color makes the matching sensors tire out and respond less. White light would normally fire all three sensors about equally, but right after a long stare the tired ones lag behind, so the leftover signal from the fresh sensors paints a colored shadow. That shadow is always the color opposite the one you stared at, and it fades over a few seconds as the tired sensors recover.
What people get wrong
Lots of people think the eye briefly stored a photo of the shape, like a tiny camera, so the ghost should be the same color you looked at. It isn't a saved picture at all. The ghost shows up in the opposite color because it comes from which sensors got tired, not from any image being kept. If your eye really stored a snapshot, the afterimage would match the original color instead of flipping to its complement.
The catch
Tiring out your sensors sounds like a flaw, but it is the same trick that lets your eyes work across a huge range of light. By dialing sensors down when a color or brightness stays steady, your eyes keep the rest of the scene readable instead of being blinded. The cost is these short-lived ghosts and a few seconds where colors look off — a small price for eyes that adjust to almost any light.
Questions kids ask
Why is the ghost a different color than the shape I stared at?
Staring tires out the sensors for that one color. On a white wall the fresh sensors fire more strongly than the tired ones, so the leftover balance reads as the opposite color — red leaves cyan, green leaves pink, blue leaves yellow.
Is my eye taking a tiny photo and playing it back?
No. If it were a stored photo, the ghost would be the same color as the shape. It comes back in the opposite color, which proves it's just which sensors got tired, not a saved image.
Why does the ghost fade away after a few seconds?
The tired sensors slowly recover and start firing normally again. Once all three kinds are back in balance, white looks white again and the colored ghost disappears.
Why does a camera flash leave a ghost that lasts so much longer?
A flash is extremely bright, so it tires the sensors far more strongly than a poster does. The harder they're worn down, the longer they take to recover, so that ghost lingers and seems to follow your gaze.
Talk about it
- Ask them: the ghost is a different color than the shape you stared at — so if your eye didn't save a picture, where did that color come from?
- Ask: why do you think the ghost slowly fades away instead of staying forever?
For grown-ups
This is a negative afterimage produced by cone adaptation. The retina's three cone types (L, M, S — long, medium, short wavelength) reduce their response to a steady stimulus through local light adaptation and some photopigment bleaching. After fixating on a saturated red, the L-cones are desensitized in that region; on a neutral white field the relatively fresher M- and S-cones dominate the local signal, and the opponent-process visual system reads the complementary color (cyan). Because the effect tracks the cones' recovery, it fades over seconds, and because it is driven by which cones are fatigued rather than any stored image, the afterimage is always the complementary color.
Keep going
What else makes you wonder?
- If staring at red leaves a cyan ghost, what ghost would a green traffic light leave when you look away?
- Why does a bright camera flash leave a ghost that follows your eyes around the room for much longer?
- Could you 'paint' a hidden picture that only shows up as a ghost after someone stares at the colored version?