Hotplate Mirage

What it shows:

A beam of light is distorted due to turbulent convection currents in air. This is a model of atmospheric distortion that affects seeing conditions in ground based optical and infrared astronomy.

How it works:

Turbulent air is provided by an electric stove ring, that heats the air above it as the warm earth dues to air sitting above it. The turbulent currents set up alter the refractive index of the air in a disordered and rapidly changing way. Light from a point source passing through these conditions is blurred and spread out.

This effect is modeled by passing a 5mW HeNe laser beam over the surface of the hotplate. Its pencil beam of light becomes distorted and the sharp spot formed on a wall is blurred and fidgety.

figure 1. laser beam passing through atmospheric distortion
hot plate

Setting it up:

You'll want the laser beam to pass a few centimeters above the hotplate element; both can be mounted on a lecture bench, separated by about a meter. The beam can be targeted on the side hall wall, or a projector screen.


The best seeing conditions for ground based optical astronomy is a resolution of just under an arc second. The Hubble Space Telescope is one solution to an unsettled atmosphere, another is the use of adaptive optics, where a laser beam is used as an artificial star to compensate for atmospheric turbulence. A deformable mirror adjusts its shape in response to the deviations of the laser beam, correcting the image of the star at the same time (see References).


G. P. Collins, Physics Today p.17 (February 1992)