In optics experiments we assume that the sun rays are parallel

Main Question or Discussion Point

Even though in optics experiments we assume that the sun rays are parallel, we can clearly see that the sunlight seems to diverge from the sun when coming through the clouds. Whats the reason of this discrepancy?

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jtbell
Mentor
Take a sheet of paper and draw two straight lines across it that are nearly parallel, but meet near one edge of the paper. Now hold the opposite edge of the paper near your eye, and tilt the paper so you're looking across the width of the paper towards the point where the lines meet. What does the angle between the lines look like now?

Or take a pair of chopsticks (or something similar), hold them together at one end with a small angle between them, and then look along the sticks from the "open" end. (Be careful not to poke your eyes out! :yuck: )

Take a sheet of paper and draw two straight lines across it that are nearly parallel, but meet near one edge of the paper. Now hold the opposite edge of the paper near your eye, and tilt the paper so you're looking across the width of the paper towards the point where the lines meet. What does the angle between the lines look like now?

Or take a pair of chopsticks (or something similar), hold them together at one end with a small angle between them, and then look along the sticks from the "open" end. (Be careful not to poke your eyes out! :yuck: )
Well, if the lines are nearly parallel how will they meet ?

jtbell
Mentor
If the lines are exactly parallel, they never meet, of course. If they are not exactly parallel, that is, with a small angle between them, then they can meet.

Light rays from the sun are clearly not exactly parallel, because they do meet at the sun!

Have you tried my little experiment yet?

Also note that even lines that are exactly parallel can appear to meet "at infinity" because of geometric perspective effects. Have you ever looked down a long straight road or railroad track that extends to the horizon on a flat plain? The sides of the road or the rails of the track appear to meet at the horizon.

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Integral
Staff Emeritus
Gold Member
Even though in optics experiments we assume that the sun rays are parallel, we can clearly see that the sunlight seems to diverge from the sun when coming through the clouds. Whats the reason of this discrepancy?
Perhaps you "clearly see that the sunlight seems to diverge" , I do not. What is this divergence you claim to observe. keep in mind that the angle at which the sun's rays come through the clouds is determined by the season and time of day, but that is not the same as divergence of the rays. I do not observe any divergence.

Gokul43201
Staff Emeritus
Gold Member
Integral, ritwik is refering to something like this: http://www.vagabond-ireland.com/silver_lining.jpg [Broken]

The answer, ritwik, is in jtbell's post, and it's the same reason that railway tracks or the sides of a straight road appear to converge at a distant point (or diverge as they approach you) - perspective!

PS: For some reason, I missed the last part of jtb's second post, and the similarity of content is purely coincidental!

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If the lines are exactly parallel, they never meet, of course. If they are not exactly parallel, that is, with a small angle between them, then they can meet.

Light rays from the sun are clearly not exactly parallel, because they do meet at the sun!

Have you tried my little experiment yet?

Also note that even lines that are exactly parallel can appear to meet "at infinity" because of geometric perspective effects. Have you ever looked down a long straight road or railroad track that extends to the horizon on a flat plain? The sides of the road or the rails of the track appear to meet at the horizon.
Oh yes, I have understood now. I did the activity with the paper, now what should I do further?