Optical illusions appear to depend not only on the human visual system but also on human culture. For example, Westerners seem to experience the Müller-Lyer illusion more strongly and the inverted-T illusion less strongly (see pictures to the right) than other ethnic groups, especially Africans.

One possible explanation would be that Westerners live in a world where geometric shapes with right angles predominate (buildings with perpendicular lines, vertical walls, horizontal ceilings, etc.). Many experiments also show that Westerners have a very strong tendency to overestimate acute angles and underestimate obtuse ones, as if they were trying to force everything back to a right angle. And that is why Westerners are more sensitive to the Müller-Lyer illusion.

Conversely, the reason that Africans are more sensitive to the inverted-T illusion would be the geography of the African savannah, with very flat topography and almost no trees, houses, or poles. Africans would therefore be less used to judging vertical lines and hence more easily deceived by the inverted-T illusion.

Optical illusions give us a better understanding of how human visual perception works. They force us to recognize that contrary to what we might think, what we see of the world is not a simple physical record like a photograph. On the contrary, the signals that your eyes receive from the elements in a scene in front of them are often ambiguous. Your brain is constantly interpreting these signals to construct an image that makes sense to it. In fact, your brain tries so hard to make sense of everything that it often finds meaning even where there is none, thus creating optical illusions.

There are several distinct families of optical illusions.

Geometric illusions are produced by the arrangement of points, lines, and simple shapes in ways that make you misinterpret these elements when you see them. Many geometric illusions involve two objects that are actually identical but look different because of their surroundings.

One of the optical illusions that has been most studied was created by German psychiatrist Franz Müller-Lyer in 1889. Though the line between the arrows on the left looks longer than the one on the right, the two lines are the same length. (If you don't believe it, place your mouse cursor over the picture!)

The inverted-T illusion: the vertical line looks longer than the horizontal one but is actually the same length. This illusion is thought to result from two factors: first, the eyes scan horizontal lines more easily than vertical ones, and second, the vertical line divides the horizontal one into two smaller segments.

In artistic optical illusions, it is not that the human visual system interprets reality incorrectly, but rather that the reality itself is deliberately ambiguous. Using various tricks of drawing, the artist creates an object that looks realistic but could never actually be built in the real world.

A simple artistic optical illusion: the impossible triangle

Some optical illusions are caused not by the brain's misinterpreting some aspect of reality, but rather by physical phenomena that distort reality's usual appearance so that the eyes can record only the distortion. Mirages are a good example of this kind of optical illusion. A more common example is the way that the sun appears to flatten out as it sets, because its light rays are passing through a thicker layer of atmosphere to reach your eyes.