Did you see a coloured square when you closed your eyes?

Colour After images

Did you ‘see’ a coloured square when you closed your eyes?

Was it a different colour?

Why can you see a coloured square when your eyes are closed?

The ability to ‘see’ anything comes from the brain when it interprets signals sent to it from the eyes. This means that there were still signals being sent even when the eyes were shut. This is a type of ‘after vision’.

What is sending the signals?

At the back of the eye in the retina, there are cells that recognise light and colour. These are called photoreceptors. The cells that see colour are called the ‘cones’. These are the cells that send the signal to the brain. There are three main types and each responds mainly to one colour, either red, blue or green.

What happens when I look at the coloured squares

Light from the square goes into the eye and hits the retina at the back of the eye. This is where the cone cells are, the ones that can recognise different colours. These cones convert the light into an electrical signal and send this off to the brain. The brain interprets all the signals and we then ‘see’ the image. There are three types of cone cells and they react best to red, green and blue light. 

Why does the square change colour?

When you stare at the square for about 30 seconds the cones recognising the colour work hard and send signals to the brain. These cells get a bit tired, sending off electrical signals, and when the eyes are closed the cones have a rest but some of the other cones still fire of so the brain can see a colour, even though the eyes are closed.

What colours should I see?

If you stare at the red square for 30 seconds or more, the cells in your retina that respond to red get tired and fire less. When you close your eyes, or switch to looking at a white sheet, your brain interprets the signals from your eyes, subtract the red and you see green as an after image.

In this way:

These colours then fade and change as the tired cones recover.

SBM August 2018

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