# Inverse Square Law for light

## Main Question or Discussion Point

Does the inverse square law work for electromagnetic radiation?

It should only work if we strictly looks at electromagnetic radiation as 'waves'. But the photon particle isn't a wave, so how would you explain how it works with the photon? I may have misconceptet the photon, but I think that a photon is a particle.

Related Other Physics Topics News on Phys.org
Hootenanny
Staff Emeritus
Gold Member
jtbell
Mentor
If you're analyzing light as a classical electromagnetic wave, the inverse square law describes the energy flux density: the energy carried by the wave per unit area that it passes through, per second.

If you're analyzing light as a collection of many many photons, the inverse square law describes the photon flux density: the number of photons that pass through a unit area, per second.

If you're analyzing (very weak) light as a collection of a few photons, the inverse square law describes a probability density: the probability that a photon passes through a unit area, per second.

I understand, so if we are analalyzing light as a collcetion of many photons, will the amount of photons that are 'shot' out be equal in every direction?
ehm,
Is there be a randomly selected 'amount' of photons that are shot in each way, just that it seems like they are equal in every direction?

I got a bit of problem of describing my question, I hope you understand. Because if photons are not stricly waves I wonder how they works...

And if we go at an extremely long distance away from the emitting atom, where the probability is 0.1 per square meter. Would this mean that there is 1 photon per tenth meter at average?

DaveC426913
Gold Member
I understand, so if we are analalyzing light as a collcetion of many photons, will the amount of photons that are 'shot' out be equal in every direction?
ehm,
Is there be a randomly selected 'amount' of photons that are shot in each way, just that it seems like they are equal in every direction?

I got a bit of problem of describing my question, I hope you understand. Because if photons are not stricly waves I wonder how they works...

And if we go at an extremely long distance away from the emitting atom, where the probability is 0.1 per square meter. Would this mean that there is 1 photon per tenth meter at average?
You can dim down a light to the point where you are effectively counting single photons. This is how we look at quantum effects in such things as the double slit experiment.

Yes, the formula will represent the probability or frequency of photons at any given point.

(BTW, your numbers are wrong. You divided when you shoulod have multiplied. A probability of 0.1 per square meter works out to one photon for every ten square meters, not tenth of a square metre.)

Yeah, I MEANT every ten square meter, I'm just not familiar with the english definition of that..

By the way, does this mean that photons are particles? In my science book it says that it has wave and particle properties, but they went with the wave model... What is the correct one? I understand that this won't have a single answer, but I want to know what makes the scientists uncertain of this.

jtbell
Mentor
Photons are neither classical waves nor classical particles. They are objects that are fully described only by quantum field theory. They have both wavelike characteristics and particle-like characteristics.

jtbell
Mentor
I understand, so if we are analalyzing light as a collcetion of many photons, will the amount of photons that are 'shot' out be equal in every direction`?
ehm,
Is there be a randomly selected 'amount' of photons that are shot in each way, just that it seems like they are equal in every direction?
To make an analogy, it's like when you toss a coin multiple times (or multiple coins at once). You expect equal numbers to come up "heads" and "tails". However, with only a few tosses (or coins), it's very possible for them all to come up "heads." As the number of tosses (or coins) becomes larger, the percentages of "heads" and "tails" both approach 50%.

Similarly, with a light source that radiates "uniformly" in all directions, if it's very very weak, it's possible for the photons to come out more or less bunched in one direction. But if you have bazillions and bazillions of photons, as is the case with everyday light sources, you get very nearly the same number in all directions.

Last edited:
Oh, ok. Do you have a link to the explanation?

DaveC426913
Gold Member
Just Google or Wiki "wave-particle duality of light".

kk, thanks

I1 x d12=I2 x d22
where:
I=intensity d2=distance per sq. unit

Hope this works for you...that is, if the thread is still active.

jtbell
Mentor
that is, if the thread is still active.
After nearly three years, well, uh... :uhh: