Optical illusions

Why do we get fooled by these? In terms of evolution, one can explain why we see things that are not there. The usual answer is that, seeing certain things that are not actually there may actually benefit us in the long term. For example, given a certain limited amount of visual information, it may be safer to deduce that there's likely to be a predator, and act accordingly. Taking that analogy to the extreme, these illusions are really the product of statistical inferences we are programmed to make. They simply reflect the fact that we extract from the image information that is useful to us.

• Cord illusion
The cords are in fact vertical
• Cafe wall illusion
This was first noted in 1973 in 'Illusion in Nature and Art', and further described by R.L. Gregory and P. Heard, Perception 8:365 (1979), and explained by the process of 'border locking'. It's named after a cafe in Bristol with a tiled exterior.
• Hermann Grid
You see light- or dark-coloured areas at the grid intersections.
• Hermann Grid (inverse)
The Hermann grid can be explained by lateral inhibition of visual receptive fields.
• Scintillating Hermann
"Count the dots!" The scintillating grid cannot be explained by Lateral inhibition alone. Observe that the spots also flicker during eye movements!
• Scintillating Hermann (inverse)
• Zöllner illusion
This is one of a family of illusions based on acute angle misperception.
• Mach's Bands
Sorry this is not actually the correct illusion.
• Hering Illusion
This shows the illusion of how acute angles are expanded locally.
• Clashing gradients
This is a variation on the cafe wall illuson.
• DeValois illusion A and B
The grey squares in A and B are of the same luminance, but because of the contrast of the surrounding squares, they appear different.
• Pandanus illusion
A two-dimensional version of the Clashing Gradients.
• Benary Cross
Notice that the two triangles are surrounded by exactly the same colours on all sides, yet they appear different colours!
• Snake illusion
This is a powerful illusion showing the effect of global factors on contrast perception - in this case, the continuity of the snake lines causes the two diamonds to be perceived as different colours.
• White's Effect
The grey bars surrounded by more white actually look lighter, contrary to colour contrast theory.
• Argyle Illusion
The middle column of diamonds is the same colour as the ones either side, though it looks much darker.
• Benussi Circle without lines
and with lines
• Benussi Square Wave square wave
and square wave with 3rd harmonic The thin strip is a simple square wave!