When light waves pass from water to air, the rays are bent, yielding the impression that something in the water is closer to us than it actually is. The key to this bending phenomenon of light, called “refraction,” lies in the difference in density between water and air.

When light passes through air masses of different densities, the same bending occurs, though to a smaller degree. Heat from the earth sometimes warms a layer of low – lying air, decreasing its density. The difference in density between the warm air close to the ground and the rest of the air above it causes light to bend and play tricks on our eyes.

Halos, Sun Dogs, and Sun Pillars

When light passes through tiny ice particles—smaller than about twenty-millionths of a meter—the crystals act like prisms that refract the light about 22 degrees. The result is a white halo circling the sun or moon. When the ice particles are slightly larger, with a hexagonal pencil shape, an arc at 46 degrees from the sun or moon can appear.

When large ice particles—about thirty-millionths of a meter—are flat and platelike, they tend to orient themselves horizontally as they fall to earth. Sunlight passing through them bends about 22 degrees, and when the sun is near the horizon, a person on the ground or in an airplane can sometimes see sun dogs, or “mock suns,” on either side of the real one.

One optical phenomenon that is caused by light bouncing off of an object—reflection—rather than light bending an object through an object—refraction—is the “sun pillar.” When the sun is low to the horizon, small platelike horizontal ice crystals sometimes reflect sunlight off their bottom surfaces. The result is a pillar of light that appears to rest on top of the sun.


On Course

If you’re ever lucky enough to see a "tertiary" rainbow, the kind that have three distinct rings, you’ll never forget it The only j tertiary rainbow I’ve seen I occurred in New Mexico when ‘ my wife and I were driving from Albuquerque to Santa Fe. Against a backdrop of towering black clouds to the east, we saw a bril­liant rainbow caused by late afternoon sun in the west After a minute, a rare secondary rain­bow appeared. Finally, a tertiary rainbow appeared. It was a vision of nature we’ll always remember.

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