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Incredible Illusions

Two Grey Steps

This illusion is similar to the Horsetail Illusion. All these illusions work because of the way your eyes see colours and edges.

You probably decided that the right hand side looked darker than the left hand side. In fact, both are exactly the same shade of grey, except for a small area in the centre. Here the greys change gradually on each side, but meet at the centre with a sharp change of colour.

Your brain pays a lot of attention to edges, which are often marked by sudden changes in colour or brightness. The "edge" in the middle of the rectangle is an important part of what your brain sees, because there is a sharp change of colour there. Because the colour changes further out from the centre are gradual, your brain doesn't notice them. You "see" the whole right hand half as if it were dark, while the whole left hand half looks lighter.

The brain often sees these kind of differences when there are none. This is because when we look at something, we want to be able to tell the bits of it apart as easily as possible. To help us do this, the brain makes any differences it finds even bigger. Sometimes this means the brain will make things look different even though they are the same.