


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?

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

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? "
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.
Mark Barendt, Beaverton, OR
"We do not see things the way they are. We see things the way we are." Anaïs Nin

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.

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 shootthrough'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.
Newish convert to film.
Pentax MX for 35mm
Bronica ETRS for 645

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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".
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 inversesquare law is actually more "realworldaccurate" 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 inversesquare law in a practical way, you usually have to get unreasonably close to the light sourceI 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
Nathan Tenny
San Diego, CA, USA
The lady of the house has to be a pretty swell sort of person to put up with the annoyance of a photographer.
The Little Technical Library, _Developing, Printing, And Enlarging_

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.

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
Mark Barendt, Beaverton, OR
"We do not see things the way they are. We see things the way we are." Anaïs Nin

As the distance approaches zero, the intensity asymptotically approaches infinity. You can never reach zero distance.

Originally Posted by Prof_Pixel
As the distance approaches zero, the intensity asymptotically approaches infinity. You can never reach zero distance.
But still this contradicts the idea of a limited intensity. The problem seems to be in the definition of intensity.
Last edited by AgX; 03072014 at 01:16 PM. Click to view previous post history.

