PURPOSE: To demonstrate how an optical mirage is created.

DESCRIPTION: A laser beam is spread into a horizontal line by a cylindrical lens, and passes over a current-carrying wire aligned along the original laser beam. The hot wire causes the air to have a very strong decreasing temperature gradient a long the central section of the laser light line, so that section bends upward. The bent laser line is displayed on the wall or screen across a long distance (perhaps the width of the room), as seen below.

Upward bending of blue light from the sky as it propagates along sun-heated sand in the desert causes blue light to appear to be coming from the ground, creating the classic illusion of a lake in the desert.

SUGGESTIONS:

See Question of the Week #210 for information on using this demonstration to enhance class involvement.

REFERENCES: (PIRA unknown.)

EQUIPMENT: Laser with cylindrical lens, and hot wire with variac power supply.

SETUP TIME: 5 min.