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  Hard shadows in real life  (Read 3811 times)
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Offline Archive
« Posted 2016-11-03 16:15:14 »

I was thinking the other day and it occurred to me that a perfect hard shadow is impossible in real life due to the fact that:

lim      sqrt(dx^2 + dy^2 + dz^2) != 0
dx->0
dy->0
dz->0

dx, dy, dz being the difference between the shadow caster's sample point and the potential shadow sample point.

and dx ≠ 0 and dy ≠ 0 and dz ≠ 0 because then the shadow caster and the sample point would be at the same position.

Essentially, the idea is that a perfect hard shadow requires the shadow caster and the sample point to be infinitely close to each other

is this correct or is this some garbage that i thought up that has no meaning lol

Offline Riven
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« Reply #1 - Posted 2016-11-04 21:26:18 »

You assume perfectly radial attentuation: photons moving apart.

Now think lasers.

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Offline Archive
« Reply #2 - Posted 2016-11-04 22:13:25 »

So a perfectly hard shadow can exist?

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Offline Riven
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« Reply #3 - Posted 2016-11-05 09:12:40 »

If the photons do not diverge, there is nothing smooth about shadows.

If the sun would be a massive grid of parallel lasers pointing at earth, and we would have no athmosphere to scatter the photons, each part of any object on earth would only receive light from one of these laser beams, which would make them either fully illuminated or fully occluded.

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Offline Archive
« Reply #4 - Posted 2016-11-05 16:50:57 »

oooo ok very interesting thank you Riven

Offline Kefwar
« Reply #5 - Posted 2016-11-05 18:22:06 »

Actually, true hard shadows should be impossible due to light diffraction close to the edge of the surface. Photons also have wave characteristics.

Offline basil_

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« Reply #6 - Posted 2016-11-05 19:32:26 »

so the smalles penumbra is .. 1 x planck length .. but still no "hard" shadow.
Offline Riven
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« Reply #7 - Posted 2016-11-06 11:09:00 »

Actually, true hard shadows should be impossible due to light diffraction close to the edge of the surface. Photons also have wave characteristics.
Sure, I realized that, but wasn't sure it was in the scope of this question - this seems like a lame excuse, but it's true Smiley

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