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How does an optical illusion work?

By , 20 June 2013

Our eyes play a huge part in helping us explore the world around us, but occasionally they play tricks on us. In this optical illusion, we’re testing your perception of colour by making an after-image optical illusion. In other words, we’re tricking your eyes to see colours that aren’t really there. Check it out!

Video transcript available here.


Image: Salk Institute for Biological Studies

How can we change the way you perceive colour? The answer lies on your retina, at the back of your eye, in the form of rod and cone cells. There are around 120 million rod cells and 6 million cone cells in the retina. A cone cell is responsible for colour vision. Signals from the cones are sent to the brain which then translates these messages into your perception of colour.

Of the cone cells there are three different varieties that pick up different wavelengths of light: red, green and blue. The colours in images we see are made up of a combination of red, green and blue.

In this video, when you stare at the negative photograph your cone cells receiving the brightest part of the image start becoming less sensitive, and the signal from those cells starts to become weaker. When the image is replaced with a black-and-white version, it appears to be coloured. The now less sensitive cone cells are sending out a weakener signal; at the same time the cones not simulated by the original image are sending a comparatively strong signal. Our brain is receiving a negative image of the original negative image, and so it now appear naturally coloured. There’s even more information and curriculum links for teachers here.

Do you have a favourite optical illusion? Let us know in the comments.

This video is the first in our 7 part series where we will share DIY experiments you can do at home. We’ll be posting a new video every Thursday.


  1. Reblogged this on News @ CSIRO and commented:

    This is quite a freaky (but interesting) experiment with colour from our education team. You may never trust your retina again…

  2. That felt soooo weird

  3. Oh now that was disturbing!

    I’m going to disturb the kids now…

    Great work guys!

  4. Arg. WordPress just ate my comment!

    Great and disturbing video. I am off to freak out the kids.

    Congrats to the Sydney team.

  5. Great Optical Illusions That Prove You Can’t Trust Your Own Mind!!!!

  6. my csiro curved cards makes me think one is bigger and one is smaller and when i swap them, the bigger oneis smaller and the smaller card is bigger

    1. ok got it

  7. i want more videos,can’t you post one each day.


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