What is the mirage effect?

Asked by: Ali Reza Shokati

Answer

A mirage is not only seen by thirsty desert wanderers. You can also see one driving comfortably along a black top highway on a sunny day. That 'wet' looking spot on the road up ahead could actually be a mirage that stays in the distance, as impossible to reach as a rainbow.

Normally, water in the distance is seen by its ability to reflect light off of its surface. In the case of a mirage, it is REFRACTION, not reflection, creating a similar effect. The hot surface warms the air immediately above it to a higher temperature than the air higher up. A light ray grazing the surface under those circumstances is bent, or refracted, upward. That's because light travels faster in warmer, less dense, air than in denser cold air. The faster motion of the lower part of a light ray speeds ahead of the upper part, causing it to bend (refract) upward.

When the refracted light ray meets your eye, it appears to be coming from the road surface instead of the distant sky. A reflected light ray follows a similar path, so the refracted ray is interpreted as a reflected one and a 'Mirage' is seen.

Reference:

• Mirages in Finland
  Great collection of mirage photos
• An Introduction to Mirages
  Great page by Andrew Young

Answered by: Paul Walorski, B.A., Part-time Physics Instructor