inverse ssquare law

[StoneNYC]

Actually, the answer to everything is 42.

+1

 

[Truzi]

Stone:

Take some university physics courses and you will find that what you say here isn't actually true.

Or for that matter, watch a few Star Trek episodes .

Of the Original Series... the subsequent series played kind of loose with science (worse than the original, I should say).

 

[fretlessdavis]

SPL can be measured at any distance. It's usually one metre and one watt to work out speaker efficiency.


Steve.

Sound waves propogate much like light does. True it can be measured at any distance, like light can, but for larger sources, 1m is pretty standardized at any power level to check SPL. A speaker is a larger source, so getting closer and closer you're not getting the full output of the speaker-- just like light.

 

[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: "

 

[fretlessdavis]

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: "

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

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.

When learning physics, early on you learn to 'deal with it', later on you learn why, and even later on, you basically throw everything out of the window and start fresh. At least that's how my education was... from basic mechanics, electricity, light, etc, then up through Quantum Mechanics. I never went further than with light and optics than a good engineer would, though.

The law works best for comparing relative distances as it falls off, but there's too much difference in light sources as you get really close to them, not to mention your measuring area would have to be so tiny, it would have little real world application.

 

[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.

 

[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?

 

[RalphLambrecht]

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

 

[markbarendt]

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.

 

[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.

 

[fretlessdavis]

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.

 

[ntenny]

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 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

 

[RPC]

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.

 

[markbarendt]

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

 

[Prof_Pixel]

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

 

[AgX]

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.

 

[MattKing]

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: "

My god, the clarity of that answer makes me flashback to first year physics!!!

 

[fretlessdavis]

But still this contradicts the idea of a limited intensity.

The law is good for true point sources-- in the 'perfect physics world' a true point source would occupy no space, so therefore, at the source, the light *would* be infinite. This can't happen in real life, but the law still applies, as the margin of error is small enough at reasonable distances, for normal everyday light sources. Just like distance can't be zero, as the sensor would occupy the same space, which according to the formula, would be 0, as would be the size of the area of measurement.

Just because the perfect parameters are impossible in practice, does not mean the law doesn't apply to average, everyday light sources.

How many people commenting have a background in Physics? Just curious.

 

[fretlessdavis]

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

-NT

Hehe:

http://xkcd.com/435/

 

[StoneNYC]

Hehe:

http://xkcd.com/435/

 

[StoneNYC]

"That's one of the problems in physics, the electron is supposedly a point particle, and the charge self repulsion energy is greater than the mass of the particle itself, whenever you deal with point entities and forces or fields you can get infinities which make no physical sense, in other words physical solutions always have a radius".

He additionally said...

"The 1 / R^2 rule does not apply when one goes down to single photon interactions."

Was his second response ...

 

[fretlessdavis]

"That's one of the problems in physics, the electron is supposedly a point particle, and the charge self repulsion energy is greater than the mass of the particle itself, whenever you deal with point entities and forces or fields you can get infinities which make no physical sense, in other words physical solutions always have a radius".

He additionally said...

"The 1 / R^2 rule does not apply when one goes down to single photon interactions."

Was his second response ...

Heh. Pretty much none of the laws apply when you get to the subatomic level. It's a whole new world, with it's own set of rules and laws down there...

 

[wombat2go]

Referring back to the OP - it is a good question , relevant for macro flash.
I think the Gauss Law still applies, but you have to consider the source with real dimensions.
This can be done by the integral calculus -conceptually, by applying Gauss law to many small illuminated points.

I hope my scribble is legible. It is for a 2 dimensional "flash tube" - approximately OK for a thin rectangular flash
and could be expanded to a 3D flash reflector.

 

[StoneNYC]

Referring back to the OP - it is a good question , relevant for macro flash.
I think the Gauss Law still applies, but you have to consider the source with real dimensions.
This can be done by the integral calculus -conceptually, by applying Gauss law to many small illuminated points.

I hope my scribble is legible. It is for a 2 dimensional "flash tube" - approximately OK for a thin rectangular flash
and could be expanded to a 3D flash reflector.

On the phone with Dad, can't read him your written stuff as I don't understand it, but he said he agress with what you typed.

 

[Prof_Pixel]

But still this contradicts the idea of a limited intensity. The problem seems to be in the definition of intensity.

Exactly! Intensity is in terms of power/area. As you approach zero distance the area approaches zero meaning the 'intensity' value approaches infinity. (...and not that the power value approaches infinity.)