Kid Questions: Why do we see the lights after they are switched off?

by Deb on December 5, 2011


If you stare at a light for about 30 seconds then turn it off or look away you will still see a light in front of your eyes. This is called an afterimage, or technically a positive afterimage.

In the back of your eyes are lots of little cells called cones. When certain types of light hit them they fire and send a signal to your brain. Your brain puts together all the signals to make a picture of what you are seeing. But your brain is very clever – we don’t actually see everything all the time, like a movie. Our eyes are moving around and looking at different pieces at different times, our brain smooths it out and fills in the gaps. Sometimes it can make mistakes or take a little while to catch up.

Positive Afterimages

Positive afterimages like seeing a light when it’s been turned off are a mistake by your brain. One of the things your brain does to make the picture you see look full is assume that things stay the same unless there is a signal that draws its attention that something has changed. A good signal is movement, it attracts our attention right away.

When you flick off a light switch there are no clues to tell the part of your brain putting together what you are seeing that the light isn’t there any more, so it keeps putting it in and you ‘see’ it even though it isn’t there.

Negative Afterimages

Negative afterimages are when you keep seeing something but it is the opposite colour. This is when you walk in from a bright day and everything looks dark, or when you are staring at something red and then have blue splotches. Negative afterimages are caused by your eyes and your brains together.

If you stare at something that is brightly coloured for a long time, the cones at the back of your eye get tired and stop firing as much. But even when the signal from your eyes gets tired and switches off, your brain knows the thing is still there and you keep ‘seeing’ it.

Colour dove

When you turn to look at something else, the tired cones stay switched off for a few seconds. But cones for different coloured light in the same area  that aren’t tired will start to fire because of the new light reaching them. Your brain doesn’t know that some cones aren’t signalling because they switched off, it just knows it isn’t getting a signal from them so it assumes there isn’t supposed to be a signal there.

This means the picture your brain puts together of what you are seeing is wrong, and has a dark hole in the middle of it the same shape as the bright object you were looking at first.

There are lots of illusions you can use to see this happening. Stare at the picture up the top for as long as you can, at least 30 seconds. The longer you look at it, the better afterimage you will get. Don’t try to look at details, pick a spot like the nose or the bridge of his glasses and focus on that as tightly as you can. This works because of your eyes and brain, it doesn’t need you to know what you are looking at.

Then look away at a bright white wall or piece of paper. You will see a very clear image of a man’s face. It is actually Pierre Cardin, the image is by Dimitri Parant.flag

What flag does the afterimage show you?

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