Monday, 28 May 2007
Crepuscular rays, in atmospheric optics, also known as sun rays or God's rays, are rays of sunlight that appear to radiate from a single point in the sky. These rays, which stream through gaps in clouds, are diverging columns of sunlit air separated by darker cloud-shadowed regions. The name comes from their frequent occurrences during twilight, when the contrasts between light and dark are the most obvious. Various airborne compounds scatter the sunlight and make these rays visible. The reason we see the light so defined is because of diffraction, reflection and scattering.
Crepuscular rays are near-parallel, but appear to diverge because of linear perspective. They often occur when objects such as mountain peaks or clouds partially shadow the sun's rays like a cloud cover. Three main forms of crepuscular rays are:
* Rays of light penetrating holes in low clouds (also called "Jacob's Ladder").
* Beams of light diverging from behind a cloud.
* Pale, pinkish or reddish rays that radiate from below the horizon. These are often mistaken for sun pillars.
The rays of the second and third types, in some cases, may extend across the sky and appear to converge at the antisolar point, which is the point on the sky sphere directly opposite the sun, and they are called anticrepuscular rays. Like crepusucular rays, they are parallel shafts of sunlight from holes in the clouds, and their apparently odd directions are a perspective effect.
Crepuscular and anticrepuscular rays behave in the same way. Crepuscular rays are usually red or yellow in appearance because the atmosphere acts as a giant lens, refracting low sunset rays into long curved paths passing through up to 40 times as much air than the rays from a high midday sun. Particles in the air scatter short wavelength blue and green rays much more strongly than longer wavelength yellow and red.
Crepuscular rays can also occasionally be viewed underwater. Particularly, in arctic areas, appearing from ice shelfs or cracks in the ice.