Does the grey ring grow darker?

A cream and yellow exhibit table and headboard with the title 'Does the grey ring grow darker?'. A grey square and circle sit on the table top at 45 degrees.

The brain judges the colour or shade of something based on its surroundings.

How it works

A grey ring (called a Koffka ring), is printed across two panels. When you slide one panel upwards, the grey ring splits in half and it seems to change shade. The ring’s shade doesn’t change, only your perception of its colour based on its surroundings.

Things to try or ask around the exhibit

  • Does the grey ring appear to become lighter or darker when you move the panel?
  • If the ring appears darker, what colour is adjacent to the ring?


Your brain can’t take accurate measurements of light, like an electronic light meter. Instead, your brain judges the ‘brightness’ of a shape by comparing it against its background or surroundings. So, something generally looks brighter or lighter if it’s next to something dark and vice versa. This effect is called brightness contrast. Your brain also notices borders around objects when it is trying to judge a shade. It can do this because of how cells in your retinas respond to light.

If you are in a darkened room with a tiny pinpoint of light off to your left, only certain cells or cones in your left eye’s retina are stimulated. When a stimulated cone cell fires away, neighbouring cone cells are inhibited, so they don’t fire at the same time as the first cone cell. This is called lateral inhibition. When neighbouring cones cells are inhibited, it contributes towards brightness contrast, because the apparent brightness of an object becomes more defined against its surroundings, making it appear brighter.

Finding the science in your world

If you want to make something look brighter, place it next to something that’s dark. Professional photographers take great care in arranging lighting conditions to take into consideration our brain’s need for contrast and other lighting effects.