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Science Now

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Eyes Have It

Your eyes and brain can work together to make you see things that aren't there.

Eye Tests

  1. Stare at the white cross in the middle of the squares for 30 seconds. Don't blink! What do you see when the colours change?
  2. Repeat the first test, this time holding your mouse button down on the cross when the colours change. That'll freeze the screen. Keep staring at the cross. What do you see?
  3. Keep your eyes moving by following the white dot. Now what do you see when the colours change?

Is the second set of squares coloured or not? Find out what's happening:

The Eyes Don't Have It

The second set of squares only seems coloured if you stare at the coloured squares first.

Your eyes contain colour detectors - each sensitive to red, green or blue. If you look at a grey square, all your colour detectors are working equally and balance each other.

But if you stare at a colour - for example, red - your red colour detectors soon get tired.

When you switch to looking at grey, your other colour detectors go back to work, but the tired red detectors are less sensitive and don't balance the others. The result? The grey square looks cyan - a combination of green and blue and the opposite of red.

How long does this false colour or afterimage last?

If you tried Eye Test 2, you'll know that it takes only a few moments for tired detectors to recover and for the afterimage to fade to grey.