inverse ssquare law

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• 03-07-2014, 10:27 AM
Ed Bray
Couple of things I didn't see mentioned although I may have missed them, are that it is true for all electromagnetic waves be they IR, UV, Sound, Visible Light, Gamma and X-Rays and that it is only true in an open space, if you are in a room or a place where there can be reflected waves then the results will be muddied and will likely not follow the inverse square law.
• 03-07-2014, 10:34 AM
jacaquarie
Okay try this, place a frog on the ground, the frog jumps half way to a wall. The wall is the light source and the frog is holding a light meter.
In how many jumps will this take for the frog with the light meter to reach the wall which is the light source?

No matter how many jumps the frog is only half way there.
What is the reading on the light meter?

Sunny 16?
• 03-07-2014, 10:35 AM
RalphLambrecht
Quote:

Originally Posted by StoneNYC
My dad is a real honest to god physicist... I'll ask him tomorrow...

stone,I would really appreciate the effort and his opinion;until then,thanks to the folks on APUG, I understand the paradox a bit better now:)
• 03-07-2014, 10:54 AM
markbarendt
Quote:

Originally Posted by StoneNYC
He didn't really give a detailed answer... This was his reply....

There is always a physical limitation to a light source, which itself has an associated radius. So in practice this mathematical situation never arises. But theoretically, yes.

"Stone wrote:
So, on the photo forum, this question arose....

This is confusing me

according to the inverse square lawB=I/d^2,theIllumination from a light sourcequadruples every time the distance from subject to light source is cut in half.Inconsequence doesn't that mean that the light source approaches infinite intensitywhen the distance to the light source approaches '0'?Hoew can this be?is there a flaw in the inverse square lawor is it limited to certain conditions?:confused: "

Well, because light sources aren't infinitely "Intense".

It might be easier to understand by starting at the light source, which has a given intensity which is the high limit (not infinity), and then doing the math as you move away from the source.
• 03-07-2014, 11:08 AM
Jim Jones
Infinitely bright point light sources are rarely encountered in practical photography. Therefore, theoretical formulas beloved by some physicists have little places in a photo forum. Our light sources are modified by reflectors and lenses, and the environment also influences the amount of light on a subject. Get practical: get a light meter.
• 03-07-2014, 11:11 AM
fretlessdavis
Quote:

Originally Posted by Jim Jones
Infinitely bright point light sources are rarely encountered in practical photography. Therefore, theoretical formulas beloved by some physicists have little places in a photo forum. Our light sources are modified by reflectors and lenses, and the environment also influences the amount of light on a subject. Get practical: get a light meter.

Get both!

One reason I switched from shoot-through's to reflective umbrellas for general use is that it controls spill and reflections better, so I can actually estimate flash exposure moving flashes around. Even if the 'perfect' light source isn't encountered in photography, it's important to understand how/why light behaves like it does.
• 03-07-2014, 11:33 AM
ntenny
Quote:

Originally Posted by fretlessdavis
It's limited by the real world application of the math.

One of my undergrad math professors used to say "The real world is a poor approximation to mathematical truth".

Quote:

In Physics, theories and laws seem to be based off of perfect conditions. There is no true point light source possible-- as it would occupy no space. As mentioned before, other effects start happening with different light sources as you get really close. There is no problem with the law, but perfect conditions for it are never attained in real life.
Well said.

The inverse-square law is actually more "real-world-accurate" than a lot of physics; if you estimate light levels using it, in practice it will work impressively well. If you start dropping things off the Leaning Tower of Pisa and expecting them to fall as if there were no air resistance, or slide things around your desk and expect no friction, you see the limitations of those models pretty quickly. But to break down the inverse-square law in a practical way, you usually have to get unreasonably close to the light source---I mean, who wants to take a photo that has nothing in it but a light bulb?

Disclaimer: I'm trained as a mathematician, not a physicist; though if I'd had an undergrad minor it would have been physics.

-NT
• 03-07-2014, 12:32 PM
RPC
Quote:

Originally Posted by markbarendt
Well, because light sources aren't infinitely "Intense".

It might be easier to understand by starting at the light source, which has a given intensity which is the high limit (not infinity), and then doing the math as you move away from the source.

Good point. Another way of stating this idea is that at zero distance, the light intensity would not be infinite, but at MAXIMUM, and depend on the FINITE intensity of the light source.
• 03-07-2014, 12:44 PM
markbarendt
Quote:

Originally Posted by RPC
Good point. Another way of stating this idea is that at zero distance, the light intensity would not be infinite, but at MAXIMUM, and depend on the FINITE intensity of the light source.

Yep
• 03-07-2014, 12:45 PM
Prof_Pixel
As the distance approaches zero, the intensity asymptotically approaches infinity. You can never reach zero distance.
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