Understanding the Inverse Square Law for Laser/Coherent Light

In summary, the inverse square law does not completely apply to lasers and coherent light. While at short distances the coherence keeps it from spreading, at long distances the aperture causes the beam to eventually start spreading. However, the divergence of the laser beam is very small, resulting in a net divergence of essentially zero for small distances. For larger distances, the spot size is noticeable and can be measured. The laser beam's divergence is a result of its properties, not governed by r-2 laws. The r-2 law only applies to sources that emit energy into a significant portion of a sphere, which includes lasers. To determine the radius of a laser beam at a given distance and starting radius, the divergence angle must also be known. This can be
  • #1
Glenn
Does the inverse square law apply to laser/coherent light?
 
Physics news on Phys.org
  • #2
No at short distances, yes at long distances. The coherence keeps it from spreading, however, because the aperture is finite, it eventually starts spreading.
 
  • #3
It is not a matter of starting to spread. The laser beam begins diverging as soon as it leaves the output mirror. The divergence is very small, so for small distances the net divergence is essentially zero. If you measure the spot size at various distances the divergence is noticeable. It is especially noticeable over any distance out side of a lab. So the spot size from a Earth based laser on the moon is appreciable. Beam divergence is not governed by r-2 laws, it is a result of properties of the laser itself.

Now, if someone were to measure the intensity of the laser signal on the moon, and assumed that it was the result of a point source radiating equally in all directions (i.e. they applied r-2 law they would arrive at an astronomically large number for the energy of the source. One of the big advantages of a laser is the ability to put a significant amount of energy into a very narrow beam.

I believe that r-2 laws only apply to sources which emit energy into a significant portion of a sphere.
 
  • #4
Originally posted by Integral
I believe that r-2 laws only apply to sources which emit energy into a significant portion of a sphere.

Then how can we determine the radius of a laser beam with a given distance and starting radius?
 
  • #5
Originally posted by kishtik
Then how can we determine the radius of a laser beam with a given distance and starting radius?
You have to also be given the divergence angle.

And Integral, any source emits light into some non-zero solid angle -- lasers included -- and so 1/r2 is always relevant.

- Warren
 
  • #6
Originally posted by chroot
You have to also be given the divergence angle.
I had no idea about that but now I think that shouldn't be hard with the div. angle. We can solve it analitically or using direct proportion. Am I wrong?
 
  • #7
It's just middle-school geometry.

- Warren
 

1. What is the Inverse Square Law for Laser/Coherent Light?

The Inverse Square Law for Laser/Coherent Light is a physical law that states that the intensity of light from a point source is inversely proportional to the square of the distance from the source.

2. How does the Inverse Square Law affect the brightness of a laser beam?

The Inverse Square Law states that as the distance from the source increases, the intensity of the laser beam decreases. This means that the further away the laser beam is from its source, the dimmer it will appear.

3. What is the mathematical formula for the Inverse Square Law?

The mathematical formula for the Inverse Square Law is I = P/A, where I is the intensity of light, P is the power of the light source, and A is the area over which the light is spread.

4. How does the Inverse Square Law apply to laser safety?

The Inverse Square Law is an important factor in laser safety as it helps determine safe distance guidelines for working with lasers. The intensity of the laser beam decreases with distance, so the further away someone is from the laser, the less likely they are to experience any harmful effects.

5. Are there any exceptions to the Inverse Square Law?

There are certain situations where the Inverse Square Law may not apply, such as when the light is not emitted from a point source or when there are obstructions that affect the light's intensity. However, in most cases, the Inverse Square Law is a reliable principle for understanding the behavior of laser/coherent light.

Similar threads

Replies
18
Views
7K
  • Other Physics Topics
Replies
1
Views
12K
Replies
3
Views
693
Replies
5
Views
1K
Replies
18
Views
1K
  • Other Physics Topics
Replies
3
Views
3K
Replies
12
Views
289
  • Mechanics
Replies
5
Views
1K
  • Special and General Relativity
Replies
32
Views
1K
Replies
8
Views
704
Back
Top