Café wall illusion

The café wall illusion is an optical illusion, first described by Richard Gregory in 1973. According to Gregory, this effect was first observed by a member of his laboratory, Steve Simpson, in the tiles of the wall of a café at the bottom of St Michael's Hill, Bristol.

This optical illusion makes the parallel straight horizontal lines appear to be bent.

To construct the illusion, alternating light and dark "bricks" are laid in staggered rows. It is essential for the illusion that each "brick" is surrounded by a layer of "mortar" (the grey in the image). This should ideally be of a colour in between the dark and light colour of the "bricks".

References

• Gregory RL, Heard P (1979). "Border locking and the Cafe Wall illusion". Perception 8 (4): 365–80.

External links

• Interactive version of the Café wall illusion (Requires Shockwave browser plugin)
• Another interactive version (Requires Java-capable browser)
• An animated "proof" that the horizontal lines are parallel and straight (This gif should open in most browsers)