One of the most glorious sights in our skies are the beams of light that stream out from clouds as if they were sent from Heaven.
These light rays often take a subtle beautiful golden color in out late afternoon sky as they fan out from between the clouds. The light beams are called "crepuscular rays" and they are yet another optical effect that is very fascinating.
The rays are visible because of dust in the air that reflects the light from the sun. The golden color is due to late afternoon or early morning filtering of the incoming sunlight as it travels through more of the atmosphere (due to the low angle of the sun).
The more atmosphere the light has to traverse, the more the shorter wavelengths are filtered out by the dust, leaving only the longer yellow, orange and red light. The light rays seem to fan out from a central point, but it is really just an illusion. The light rays are actually parallel!
This illusion is due to the fact that the source of the light (the sun) is so far away, that the light essentially starts from an infinite point. The best way to think of this is to envision a pair of railroad tracks.
If you look way down the tracks into infinity, the tracks seem to converge in the distance. Of course they do not, they remain parallel, but to our eye it would appear that the tracks are coming together at a point way in the distance.
The same illusion is what creates the fanning out of the crepuscular rays.
Another phenomena that is seen on rare occasions are "anticrepuscular rays." These take a little imagination to figure out. If the sun has already set from your vantage point (or has yet to rise), you may see beams of light seeming to come together along the opposite horizon (180 degrees from the sun).
(PHOTO: Millie Franco)
This is caused by the sunlight and shadows cast by clouds in front of the sun. At the cloud level (higher in the sky) the sun has already risen or set relative to the horizon. The light from the sun shines on to the clouds, casting parallel beams of light and shadow across the sky. In the distance behind you, those beams seem to converge just like the railroad tracks!
