Optical illusion – the Adelson checker shadow illusion

Optical illusion - the checkerboard shadow illusion

Checkerboard shadow optical illusion.
In the top photograph, which square is darker – 1 or 2?
Answer: neither, they are the same. This is shown in the second figure, where a uniform gray strip crosses the image to show comparisons in tone between different squares.
This image is a photograph – the illusion works with the real objects, an apple on a checkerboard, and is not affected or caused by the fact that this is a two dimensional image

The checker shadow illusion is associated with the work of Edward H. Adelson

This is how the illusion works.
An object on a checkerboard casts a shadow on the board.
The parts of the board in the shadow are darker than the parts of the board that aren’t.
The human brain knows that the squares on the board are all same – either white or dark gray, so it compensates for the darkening effect of the shadow, making the white squares that are in the shadow seem lighter than they really are (which, if you get the light levels right, can be as dark as the dark gray squares).
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