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Looking For Sunbeams - Page 2

© Keith C. Heidorn

A beam's contrast also depends on the density of scattering particles, the scattering angle of the sun, and the line-of-sight distance through the light ray. The highest degree of contrast occurs when the rays are viewed toward or away from the sun rather than when viewed at right angles to the beam, such as when they pass overhead.

The pattern recognition functions of our brain may also cause us to perceive "dark rays." These are usually seen when scattered clouds intercepting portions of the sun's radiance cause us to interpret their shadows as "rays" rather than the bright sky patches between the shadows. The accompanying figure showing alternate bands of light and dark illustrate this point. Look closely at the bands, are they light on dark, or dark on light? Your perception may jump between both visualizations.


Another illusion arising from sunbeams is the perception that they converge on the solar disk. In fact, light rays emanating from the sun are parallel when they strike the Earth. That sunbeams appear to converge on the sun is due to perspective -- the apparent convergence of parallel lines at some distant vanishing point. This is the same visual effect that causes railroad tracks or telephone lines along both sides of a highway to appear to converge in the far distance.

The most beautiful and striking visualizations of sunbeams are the crepuscular rays seen when the sun is near the horizon. Crepuscular rays are usually red, orange, and yellow in colour because blue light is effectively scattered out by air molecules and very small particles in the sky.

Photo courtesy of NOAA/US Dept. of Commerce
Although we see crepuscular rays diverge from the sun outward toward us, if we turn around, we may see them converging toward the eastern sky. These portions of the rays that converge on the anti-solar point of the sky are called anti-crepuscular rays. Anti-crepuscular rays when visible appear as a pastel pinkish glow against the darkening blue sky.

Now that dawn comes at a more civilized hour, I have two daily chances to marvel at the beauty of crepuscular rays. Often they tinge the underbelly of a long grey cloud with a reddish orange hue called salmon, an appropriate image for those of us on the Pacific coast.

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