Hard shadows in real life

General Discussions
#1

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

#2

You assume perfectly radial attentuation: photons moving apart.

Now think lasers.

#3

So a perfectly hard shadow can exist?

#4

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.

#5

oooo ok very interesting thank you Riven

#6

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

#7

so the smalles penumbra is … 1 x planck length … but still no “hard” shadow.

#8

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 :slight_smile: