optic question

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q-w-e-r-t-y
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optic question

Postby q-w-e-r-t-y » Wed Mar 12, 2008 1:53 am

I'm not sure if this is the right place to post the topic but...

I heard that there was an optical phenomenon like the following.

You create two shadows under the Sun (or some sort of parallel light ray source).
When you bring two objects that are creating the shadows close together and
before they actually touch each other or overlap, two shadows become one.

Have you heard of this?

Does it make sense?

Can anyone explain the physics?

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quizivex
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Re: optic question

Postby quizivex » Thu Jun 12, 2008 2:35 am

To me this sounds like plain old diffraction, though I could be mistaking.

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dlenmn
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Re: optic question

Postby dlenmn » Thu Jun 12, 2008 4:47 pm

No, it's magic.

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will
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Re: optic question

Postby will » Thu Jun 12, 2008 11:12 pm

Definitely magic. There is no physics involved.

vicente
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Re: optic question

Postby vicente » Sat Jun 14, 2008 1:27 am

Shut up will and dlenmn, be serious to this guy, he's looking for our help.

It's not magic, what a stupid answer.

Clearly, it's because God made it that way.

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quizivex
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Re: optic question

Postby quizivex » Sat Jun 14, 2008 3:07 am

Lol... so far we have three different possible explanations...

maybe it's time to settle this debate with a poll?

sss1983
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Re: optic question

Postby sss1983 » Tue Jun 09, 2009 6:14 pm

im sorry sir.
but if im not mistaken the problem u told isnt real
the two sahdows shudnt touch b4 the actual objects ..........
this can be proven by simple geometry

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twistor
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Re: optic question

Postby twistor » Tue Jun 09, 2009 8:10 pm

To me it sounds like an effect of geometric magnification.

___
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Re: optic question

Postby ___ » Wed Jun 10, 2009 1:13 am

Due to the finite angular size of the sun, the penumbral shadows of the two objects will overlap before they touch, whereas the umbral shadows should remain distinct. Imagine you're sitting beneath the objects and looking up as they start to eclipse the sun. Each object will cover a separate part of the sun, so you'll be in each of their penumbrae. Thus, you get overlapping shadows from non-overlapping objects.

[Edit] Wait a second, why was this topic raised from the dead? [\Edit]

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grae313
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Re: optic question

Postby grae313 » Fri Jun 12, 2009 9:48 am

dumb question - is the penumbra just a fancy name for the effects of diffraction, or are the phenomena distinct physically?

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twistor
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Re: optic question

Postby twistor » Fri Jun 12, 2009 11:30 am

Penumbras can be caused by multiple effects. For instance, in radiation therapy one talks about geometric penumbra (due to the finite size of the photon source), transmission penumbra (because some photons can pass through collimators), and scatter penumbra (due to scattering in the patient).

Look at your shadow on the ground... it's a very sharp cutoff between the shadow and sunny part of the sidewalk. If there were a penumbra present you would see a gradation in your shadow. Since you can't see it, it must not be the effect he's referring to since his claim is that the effect is observable by the human eye. I actually don't think geometric magnification is the answer now, either, because the key is that the sun is approximating a parallel beam light source, so there can't be a geometric magnification effect. And I'm guessing any diffraction effects would be too small to be observed by the human eye also.

So until someone goes outside and tries to do this with sunlight as the source and is successful, I'm gonna say I don't believe it.

Is the original poster even around anymore? This topic is oooooolllddd.

___
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Re: optic question

Postby ___ » Fri Jun 12, 2009 5:45 pm

twistor wrote:Look at your shadow on the ground... it's a very sharp cutoff between the shadow and sunny part of the sidewalk. If there were a penumbra present you would see a gradation in your shadow. Since you can't see it, it must not be the effect he's referring to since his claim is that the effect is observable by the human eye.

Your shadow isn't fuzzy? Mine definitely is. Go outside and hold your hand about an inch off of the sidewalk. The outline should look sharp. Now raise it up a foot or two, and unless the sun acts very differently wherever you are, the outline will get fuzzy.

grae313 wrote:dumb question - is the penumbra just a fancy name for the effects of diffraction, or are the phenomena distinct physically?

As far as I know the phenomena are distinct. The umbra is the part of the shadow in which the light source is totally obscured, and the penumbra is the part where the light source is only partially obscured. No diffraction needed. Although, as twistor mentioned, the idea can be easily generalized. Wikipedia has a good picture of the effect in relation to eclipses: http://en.wikipedia.org/wiki/Umbra




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