How do artists create optical illusions?

Answer 1

When light from a lighter medium enters a heavier medium like from air to glass the refraction occurs towards the normal. However when light passes from a heavier medium to a lighter medium the refraction occurs away from the normal. As the incident angle in this case keeps incresing the refracted light keeps moving away from the normal until it reaches a critical angle when total internal reflection occurs. Thus on a hot day when you travel on a highway that is paved, the air near the surface gets hotter and lighter than the air slightlyabove it. As we drive we can see this total internal reflection occurring which gives you the illusion that there is a water puddle at a distance when in actuality there is no water. This is a mirage and this is an example of optical illusion. The brain processes the signals and gets slightly confused and we see illusions. There are many such optical illusions in nature (and man made).

Answer 2

It is basically the way the eye sees things... and how the eye focuses.