Corona

The rings of small angular diameter, that appear around the sun or moon on translucent clouds, are called coronas. A corona is produced by a diffraction of light on cloud elements or dust particles. The ring diameter is larger for small diffracting particles than for large ones. The colors are arranged similarly to those of a primary rainbow; however, corona colors are much less developed than rainbow colors. Coronas can also be seen around street lights or the headlights of approaching cars, when looking through a dusty windshield.

Coronas are easily produced in a laboratory. In a darkened room, look toward a strong point-light source, which should be at a distance of at least 12 feet from the observer. Such a light source can be made by taping a piece of cardboard with a small hole (1/8 to 3/16" diameter) in front of a flashlight (with good batteries) or to the lens of a projector.

Breathing on a small piece of good window or picture glass or a 2 x 2" slide cover glass that has been thoroughly cleaned, and holding it close to your eye, you will see a corona. If you cool the glass in a refrigerator for a while, dew will form on it when you take it out, which will last longer than the condensation from your breath.

Excellent coronas are produced on glass dusted with Lycopodium powder (the spores of a club moss.) The spores from mushrooms, such as puff-balls, can also be used. The smaller the dust particles and the more uniform their size, the better the coronas will be developed. A bright corona is usually surrounded by additional rings.



Material: A sheet of glass, 2 x 2" or larger

A flashlight or slide projector

A small piece of cardboard, tape