Purpose: To show the bending of light due to temperature-dependent variation of the index of refraction of air.

Aim the laser just past the heating surface of a hot plate, nearly parallel to the surface. As the hot plate reaches its final temperature, the beam spot will move about 10 cm on a wall about 10 meters away.

Move the table containing the hot plate away from the beam and back. The beam spot on the wall will move. Using your breath or waving your hands, blow on the air near the hot plate. The beam spot on the wall will wiggle.

The article by K. Tennakone [Am. J. Phys. 51, 270 (1983)] gives a quantitative analysis of this demonstration.

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