Hotplate Mirage
refraction - atmospheric distortion - adaptive optics
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
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.
Comments:
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 reference).
Rating **
References:
G. P. Collins, Physics Today p.17 (February 1992)