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Optical phenomena

Find out how your eyes react to different optical phenomena and amaze yourself at how images appear to move all by themselves, and how you appear to see things that your neighbour can’t see at all.

Café wall effect

- slanted or square?

An interactive diagram:
Move the square within the beam using the yellow dot at the bottom on the right. What happens?

This phenomena was first observed on the wall of a café in Bristol, covered in black and white tiles.On viewing the café wall, the lines of mortar gave the impression that the tiles had slanting edges, and yet in reality they were all parallel and of the same size.

How can this this effect be explained?
The eye is not capable of defining the lines separating the dark and light tiles as distinct objects. Our brain, as it tries to interpret the form, cannot distinguish it as a light or dark tile and adds in the neighbouring tile too. This gives the impression that the tiles have slanting edges.


Why do stationary objects suddenly appear to move?
Try it out yourself.

The picture below is interactive:
Start the animation below. You start the spiral with the yellow arrow and stop it above the square. Look at the centre of the spiral for 30 seconds, and then look at a stationary object, for example a glass. Does the glass suddenly appear to move?

This is all to do with persistence of vision when regarding moving images, also known as the waterfall effect. Look at a waterfall for some time or any other constantly moving thing and then look at a still picture and it appears to move in the opposite direction for a while.