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Q & A: The optics of sunlight

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Most recent answer: 08/20/2016
Q:
So this is something I have pondered over the past few days since visiting the beach recently. I was looking up at the sun around mid day as it appeared behind the clouds, and I noticed that as the sun moved over breaks in the clouds, rays of light scattered through the openings,down to the ground. It appeared as if the angle of the rays were spreading out as they went towards the ground (you can see pictures of this with a quick Google search). I am a biology major so forgive my lack of knowledge in physics, but if the sun is 93,000,000 miles away, and is much much larger than the earth and moon, why is it that the rays don't all point straight down around the clouds that are covering the sunlight? I attempted to replicate what I saw by taking a peice of black paper, and poking a few holes in a concentrated spot in the center with a needle. I then took the piece of paper and suspended it a few inches above the surface of my desk. I proceeded to take a fairly bright flashlight and point it directly over the cencentraded holes on the paper and examine the beams of light that travel through. From what I could see, the little holes allowed small dots of light to form on the desk underneath the paper, but the most interesting thing about this, is that as the flashlight was moved a few feet from the holes(in an attempt to replicate the suns distance from the clouds), the spots of light on the desk remained concentrated in roughly the same shape and size as the holes the light was coming from, but when I moved the light closer to the paper, they spread out just as the sunlight does through the breaks in the clouds. The other reason I'm curious about these beams of light, is that on a semi-cloudy day I can look at the sun casting light onto clouds directly above me, but if I look to the horizon I can see the sunlight casting onto the clouds facing me. If I turn around 180 degrees and look to the horizon, the clouds are reflecting the sunlight in the same manor as the clouds on the opposite horizon. However, the clouds directly above me only reflect the sunlight from directly above, and not from the sides of the clouds towards the horizon. If the sun is (estimated according to NASA) 93 million miles away and far bigger than the earth, shouldn't all the light from the sun be focused exactly the same way across the entire side of earth facing the sun? Shouldn't all the clouds I look at when the sun is in the middle of the sky be reflecting the sunlight in the exact same fashion? There are videos you can watch on YouTube of a rocket going above all the clouds and the sun appears to be casting a hot spot (bright or concentrated light) directly onto the clouds below it, but not all the clouds on that side of the earth facing the sun. I simply don't understand how something so far away with such large mass, can only focus it's light directly below it instead of spreading it in all directions as it should. If you took the time to read this, please go outside in mid day to see what I'm talking about, it just appears as if the sun is either way smaller or closer the earth based upon the way it's light reflects onto the clouds. Thanks!
- 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? 

http://blogs.discovermagazine.com/badastronomy/2011/11/02/crepuscular-rays-are-parallel

http://www.skyandtelescope.com/astronomy-news/observing-news/earths-shadow-07292014/

It's also hard to explain anti-crepuscular rays if crepuscular rays aren't parallel. These are rays from the setting sun that extend from the west all the way across the sky to the eastern horizon, and appear to converge again at a point exactly opposite the sun—because they're parallel. There's a cool photo in the second link above. Why don't you also see the non-parallel rays not converging at the anti-solar point?

But all that aside, the perspective explanation is perfectly adequate to explain the appearance of the rays. If you need to convince yourself, you could program a simple model of the geometry with some reasonable assumptions about distances and see how it works out. Something like this: https://www.geogebra.org/m/xkPz5fhc

Rebecca H.


(published on 08/20/2016)

Follow-up on this answer.