Which row of chess pieces is darker?

A cream and green exhibit table and information panel with the title 'Which row of chess pieces is darker?'.

Your brain constantly compares how light or dark something appears against its surroundings.

How it works

Look at a picture of chess pieces and slide over a plain grey panel to see if the chess pieces change shade before your eyes. This chess piece illusion was originally developed by Professor Bart Anderson.

Things to try or ask around the exhibit

  • Which row of chess pieces is darker?
  • If shadows look like solid black shapes on the ground, why don’t you feel as though you could trip over them?

Background

This illusion reveals how your brain processes three layers of information when you’re trying to work out how light or dark something may be.

The rows of chess pieces in this exhibit are the same, but they’re placed against black fog or white fog surroundings. Because your brain compares the chess pieces to their foggy surroundings, you mistakenly (but understandingly) assume that the rows of chess pieces must be darker or lighter than each other.

The foggy surroundings make the chess pieces appear darker or lighter, because your visual system breaks the pictures down into three layers:

  • illumination (how much light hits the surface of the chess pieces),
  • reflectance (how much light the surface reflects towards your eyes) and
  • transparency (anything like a curtain, smoke, shadow or glass that the light needs to pass through).

Somehow, your brain makes allowances for these different contexts and adjusts for the conditions.

Finding the science in your world

Your visual system uses a series of filters to know that shadows are just shadows and not solid objects. Every time you look at an object, your brain assesses the illumination, reflectance and transparency of something to determine how light or dark it is. If your brain didn’t compare contrasts and boundaries between objects, you wouldn’t be able to read a book in dim light or see things under moonlight.