The Hindu : National : An optical phenomenon

Online edition of India's National Newspaper
Wednesday, Jul 28, 2004

National

National

An optical phenomenon

By Lalithasai

CHENNAI, JULY 27. A wide, hazy halo around the sun materialised here today around 11 a.m., and many a neck remained craned skywards watching the rather unusual sight on a cloudy day. It lasted about two hours. The B.M. Birla Planetarium received a number of calls. There were no reports of such a halo having been visible elsewhere in southern India.

According to P. Iyamperumal, executive director, Tamil Nadu Science and Technology Centre, Chennai, it was caused by refraction in ice crystals associated with high-altitude clouds. The inner part was reddish and the outer area bluish. It is what is termed a ``22° DEG: a ring of light formed 22° DEG from the sun (or the moon). It is formed by hexagonal ice crystals with a diameter of less than 21 micrometres. Normally ice crystals will have a refractive index of 1.31, which is a measure of light rays slowing down inside a crystal. This corresponds to the angle of minimum deviation of about 22°.

When the crystals are oriented at random, sunlight is refracted in all possible directions. But at an angle of 22°, a visible brightening occurs; at this angle the direction of the path of the light in a rotating prism changes slower than at other angles.

The angle is the angle of minimum deviation. The light is refracted when it enters the ice crystal and again while leaving it.

The two refractions cause light to bend by 22°, producing a ring.

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