Our eyes can play tricks
on the brain; sometimes it’s as ordinary as a pinstriped suit that helps make
people look thinner. Others, such as seeing a mirage in the desert, have
greater significance, even though they aren’t what they seem. At the heart of
all optical illusions is the brain’s ceaseless effort to make sense of what the
eyes are telling it. 

Anybody who has driven
in summer has probably seen a mirage. That pool of water shimmering in the
middle of the road is actually caused by the bending of light rays. The light
bends because the air near the road is a different temperature from that above
the road. Air near the road is hotter and thinner than air higher up. Light
moves faster through hot air and slower through cold, so the light near the
road ripples and looks like water.