# Q & A: The optics of sunlight

Q:
- Kyle (age 22)
Pittsburgh, PA, USA
A:

There's a lot of optics going on here!

The rays of sunlight you've observed are called "crepuscular rays." The word crepuscular refers to dawn and dusk, the times when this phenomenon tends to be most visible because of the increased contrast between sunlight and the dark sky or clouds, but crepuscular rays can be seen at any time of day.

What you're seeing are shafts of sunlight that pass through broken areas in the clouds, separated by darker unlit bands. As you noticed, the rays may appear to radiate from a point and spread out as they reach the ground, but this is actually an optical illusion. The rays are nearly parallel to each other all the way from the clouds to the ground. They appear to converge to a point in the sky because of a perspective effect—exactly the same reason that if you stare down a long, straight road, it appears to converge to a point on the horizon even through the sides of the roadway always remain parallel.

In your experiment with the holes in the black paper, I think you may have actually created little pinhole cameras, which are also a neat thing to explore. A small pinhole can act like a lens, projecting an image of an object on the other side of a pinhole. You can view the image on a piece of paper (or, in your case, your desk). I think when you move the flashlight far away, you see small spots because the pinholes are imaging a smaller-looking object, and when you move the flashlight closer its apparent size increases (think about moving something closer to a camera) and the spots get bigger. There could also be some diffraction effects going on here. Pinhole cameras are actually a great way to safely observe the sun during an eclipse without looking at it directly.

You're certainly correct that the rays of light from the sun are close to parallel by the time they reach us on Earth. I'm having a hard time understanding your description of the way you observe sunlight reflecting off clouds. But the image of the sun on the "top" of the clouds is easy to understand—sunlight is reflecting off water or ice crystals in the clouds, forming an image of the sun like the image in a mirror.

Since the rays from the sun are coming in almost parallel to each other and you can see the sun from anywhere on the sunlit side of the Earth, you might wonder why the sun looks like a round spot at all, or why the other stars look like points instead of just a uniform glow. The reason for this has to do with what the lenses in your eyes or in a camera do to light. You can read more about that in this previous Ask the Van question: http://van.physics.illinois.edu/qa/listing.php?id=28159

Rebecca H.

(published on 08/15/2016)

## Follow-Up #1: Crepuscular rays are parallel

Q:
Please see a previous repeat of this question at https://van.physics.illinois.edu/qa/listing.php?id=2029 The railroad metaphor given there as part of the answer is not applicable since the observer stands in plane of the rails and and in their middle and hance sees a fast rate of appearant convergance. If she stood above the plane she would see a slow rate of convergance. This is the case with sun beams. Hence the answer must be otherwise. I suspect that a piece of highly crystallized cloud above the cloud blanket acts as a chandelier and casts secondary beams and at the same time blocks the primary beams. I suspect we see the secondary beams as converging. On the other hand the beams that are parallel (as reported by the other van fan) are the primary beams -- without blockage by a crystallized chandelier cloud.Please expand this text box.
- Mehran (age 65)
Miami
A:

If this were true, you'd see those "secondary" converging rays in photographs taken from orbit (analogous to looking down on the railroad tracks from above). As far as I know, they not observed. The "rays" are light scattering in the air (in all directions), so why would you see them from the ground but not in photographs taken from the ISS?