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Q & A: sunbeams and perspective

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Most recent answer: 03/02/2014
Q:
Since sun is so far away, its rays come to us almost parallel. So, why then, on a semi-cloudy day, do I see sunbeams converge at a point at or above the clouds -- i.e., they are not parallel?
- Mehran (age 5)
Lisle, Illinois
A:
Hi Mehran,

Sunbeams are seen because of light scattered from water droplets and dust and smoke particles suspended in the air. If the cloud cover only has a few small holes in it, then parallel shafts of light will scatter light in all directions so you can see the sunbeams.

The light from the sun really isn't exactly all parallel because of the size of the sun. Some light rays come from one part of the sun and others from other parts, and these can have different directions. But for the purposes of your question, this effect may be neglected.

The reason you see the sunbeams converge is good old perspective -- the projection of an image of a three-dimensional object onto a two-dimensional plane. If you stand on a railroad, for instance, the rails will appear to converge the farther away you look, even though the rails are parallel. If two objects have the same distance from one another, then the angular separation between them decreases as the two objects are farther away.

The reason the convergence point is at or above the clouds is because the beams are farther away from you high in the sky than they are closer to the ground. It is conceivable that if you stand under the holes in the clouds and the sunbeams shine nearly parallel to the ground, then they will appear to converge as they go downwards, just as the rails do.

Tom

(published on 10/22/2007)

Follow-Up #1: convergence?

Q:
I don't buy your answer. Railroad rails don't appear to converge nearby, they appear to converge at a great distance. Sunbeams, appear to converge very near to the Earth, right above the clouds. This is a great question which I've often pondered myself, and it deserves better than your lame explanation which is wrong even if you insist it isn't. Try again, but go out and study them yourself before you try to offer such a simple answer to a question much more complicated than you realize. I also don't buy that the original question was submitted by a four year old, but that's not your fault.
- Dave (age 46)
A:
Is the "great distance" at which railroad  tracks seem to converge really farther than the distance to the apparent sunbeam convergence "very near to the earth"? Or have you shifted scales, so that a mile in one context seems large but in another context seems small?

Mike W.

(published on 05/24/2010)

Follow-Up #2: r-e-s-p-e-c-t

Q:
Some people just don't have any respect whatsoever!
- Anonymous
A:
Ain't that the truth.

Mike W.

(published on 05/25/2010)

Follow-Up #3: parallel sunbeams

Q:
Why do the sunbeams shown at the following web site appear to be parallel? http://www.edhat.com/daypics2011/blackwhiteclouds1.jpg
- Dan Bolinger (age 52)
Kettering, OH, USA
A:

Each sunbeam of course comes directly from the Sun, so they would all intersect back at the Sun. Although that means that they aren't strictly parallel, the Sun is so far away that they are very close to parallel on  distance scale of a mile or so. In that picture you link to the sunbeams all seem to be at about the same distance from the camera over the whole length seen, so there are no interesting perspective effects.

Mike W.


(published on 03/02/2014)

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