When a laser beam passes over a hot plate, the convection currents in the air above the plate cause the laser spot projected on a screen, to jiggle. (See video at right, above.)

Demonstration 52.11 -- Convection cell, describes the process of convection, in which heat is transferred through a medium by the motion of fluid from relatively hot regions into cooler regions. As a fluid heats, the average kinetic energy of its molecules increases, and the fluid becomes less dense. Thus, relatively hot fluid is less dense than cooler fluid, and it rises, while the cooler fluid sinks to replace it as it does so. (See demonstrations 36.34 -- diet vs. regular Coke, 36.36 -- Archimedes’ principle, 36.37 -- Cartesian divers or 36.39 -- helium-filled balloon). The hot surface of the hot plate heats the air near it, which becomes less dense than the cooler air above it, and it rises, while the cooler, denser air above it sinks. This produces convection currents near the surface of the hot plate.

The index of refraction of air is very close to one; for air at one atmosphere and 20° C, it is 1.0003. As the temperature or pressure changes, however, the density of the air changes, and thus so does its index of refraction. Demonstration 80.18 -- Refraction tank, shows the phenomenon of refraction, which occurs when light passes (at a nonzero angle of incidence) from one medium into a second one, whose index of refraction is different from that of the first medium. As noted above, the heating of the air near the surface of the hot plate produces convection currents. The changes in the density of the air that produce these convection currents also result in changes in the index of refraction of the air within them. Because of this, light passing over the hot plate encounters variations in the index of refraction of the air through which it passes, with randomly oriented boundaries for the various regions that have different indices of refraction. It is thus refracted at random angles as it passes above the surface of the hot plate. This results in a random displacement of the spot that the laser beam produces on a screen; it jiggles. The video above, at right, shows this.

This is the same phenomenon that you observe, for example, when you view objects along a line that is near a hot surface, such as a road on a hot day, or when you view objects through the exhaust plume from a truck. On a larger scale, a similar phenomenon causes stars to twinkle, and blurs the images formed by telescopes. Temperature gradients in the atmosphere, and the resulting air currents, cause fluctuations in the index of refraction along the path via which light from celestial objects reaches us at the earth’s surface.

References:

1) Halliday, David and Resnick, Robert. Physics, Part Two, Third Edition (New York: John Wiley and Sons, 1977), p. 939.