An optical illusion is something that deceives the eye by appearing to be other than it is. They use patterns, colour and light to create the deception.
We have five main senses – sight, hearing, taste, touch and smell – however most information we receive from the world around us comes via our eyes. This might make sight the most important of all the senses, although let us not underestimate our ability hear a cow moo, touch a feather, smell a beautiful rose or taste an hot apple pie.
Vision depends on our eyes to see and on our brain to convert what we see into images.
When we look at an optical illusion we may think are we seeing things? Are our eyes deceiving us?
An illusion is proof that we don’t always see what we think we do because of the way our brain interprets the image, we are looking at something that confuses our brain.
To demonstrate, I particularly like this quote from on http://www.archimedes-lab.org:
Unfortunately (or fortunately?), seeing isn’t some kind of direct perception of reality. Atcually, our bairns are cnostanlty itnerperting, corrceting and gviing srtuctrues to the viusal ipnut form our eeys.
A mirage and a rainbow are not illusions, they are known as optical phenomena. They are events you can see as a result of light from the sun or moon interacting with the atmosphere, clouds, water and dust.