Does the Inverse Square Law Apply to Lasers and Perfect Lasers?

In summary, the conversation discusses the application of the inverse square law to lasers. While it does not apply in a perfect laser beam due to the lack of beam divergence, it does hold true for real lasers with some divergence. However, this far-field divergence angle depends on the wavelength and minimum spot size of the beam. The conversation also touches on the use of lasers as weapons in space and the use of mirrors to reflect laser beams.
  • #1
CINA
61
0
I've read from several sources that says the inverse square law doesn't apply to lasers, but I've been told that it does apply. Which is right?

Also if it does apply in today's lasers what about in a perfect laser?

Thanks.
 
Science news on Phys.org
  • #2
In a perfect laser beam the [itex] \frac 1 {r^2} [/itex] law would not hold. The beam diameter would not change, you would have the same energy density at any point on along the beam.

HOWEVER. Perfect laser beams do not exist. Every real laser has a bit of beam divergence. If the beam is divergent, then at a significantly large distance you would measure a drop off in energy density according to [itex] \frac 1 {r^2} [/itex].
 
  • #3
How long is a significant distance? 1 AU? I ask becuase I was discussing the application of lasers as weapons in space.
 
Last edited:
  • #4
Normally "perfect laser" doesn't require the wavelength to be so negligible compared to the beam waist..
 
  • #5
Energy falling off "as 1/r2" is true for light expanding from the source in a sphere- the energy is spread across the entire surface of the sphere which area is increasing as r2.

If by "perfect laser" you meant "no spread at all", then the energy would not drop (except of course by absorption by space dust or whetever material was in its way).

If your laser has a spread angle of [itex]\theta[/itex] (That would be a three dimensional "dihedral" angle) then the area over which it is spread at distance r from the origin would be [itex]\theta r^2[/itex] and so the strength would still fall off at a rate proportional to r2 (but the proportionality would be small for very small angles).
 
  • #6
Whats Known About Very Small Return Waves Riding Back On Em Waves Or On Any Other Wave?
 
Last edited:
  • #7
I have a really good book about science for SF writers, written by Ben Bova (who is both a real scientist and a novelist). The section on lasers in space indicates some counter-intuitive behaviour. Something about the beam remaining coherent for quite some distance, then diverging at some rate that doesn't follow the inverse law.
I can't remember the details, and the book is back at my house. I'll try to find it after work and get back to you.
 
  • #8
The inverse square law does not hold true because the inverse square law assumes the source is radiating isotropically. Lasers do however have a far-field divergence angle which can be used to calculate the irradiance profile some distance away. This far-field divergence angle depends on the wavelength of the laser and the minimum spot size of the beam.

Claude.
 
  • #9
I ask becuase I was discussing the application of lasers as weapons in space.
On one of the Apollo missions, they left an array of mirrors, with each shaped like a cube sliced in half diagonally, so they would reflect from any direction. Earth based lasers can be pulsed at the mirrors, and it takes a bit before the reflection comes back.

http://science.nasa.gov/headlines/y2004/21jul_llr.htm [Broken]

One of the advantages of being an old guy (55 years old), is being able to remember this stuff. At the time, major networks included snippets of videos of the laser beams being bounced off the mirrors.
 
Last edited by a moderator:
  • #10
Claude Bile said:
Lasers do however have a far-field divergence angle which can be used to calculate the irradiance profile some distance away. This far-field divergence angle depends on the wavelength of the laser and the minimum spot size of the beam.

That sounds familiar. I never got the chance to go back to the house today, but this might very well be what Bova was referring to.
 
  • #11
Jeff Reid said:
One of the advantages of being an old guy (55 years old)


Shouldn't you be dead by now? Bloody hell, but you're ancient!
 
  • #12
if you take the average on the whole spherical surface at a distance r, even for lasers, perfect ones, the inverse square law will hold. On average, though.

best wishes

DaTario
 
  • #13
Jeff Reid said:
On one of the Apollo missions, they left an array of mirrors, with each shaped like a cube sliced in half diagonally, so they would reflect from any direction.
Interestingly, these are the same type of things used on the masts of sailboats to make them more visible on radar.

http://www.marisafe.com/Store/viewItem.asp?ID=108010869&CID=10800000&FLT=108010869
 
  • #14
DaTario said:
if you take the average on the whole spherical surface at a distance r, even for lasers, perfect ones, the inverse square law will hold. On average, though.
Huh? That can also be said of a single cannonball, which is exactly the case for which you would prefer to say that the inverse square law doesn't apply.
 

1. What is the inverse square law?

The inverse square law is a physical principle that states that the intensity of a physical quantity, such as light or sound, is inversely proportional to the square of the distance from the source of the quantity.

2. How does the inverse square law apply to lasers?

The inverse square law applies to lasers because the intensity of the laser beam decreases as the distance from the laser source increases. This means that the further away an object is from the laser, the weaker the laser beam will be when it reaches the object.

3. Why is the inverse square law important in laser safety?

The inverse square law is important in laser safety because it helps determine the safe distance for using lasers. As the intensity of the laser beam decreases with distance, the risk of eye or skin damage also decreases. Therefore, understanding the inverse square law is crucial in ensuring safe laser use.

4. Can the inverse square law be applied to all types of lasers?

Yes, the inverse square law can be applied to all types of lasers, including continuous wave lasers, pulsed lasers, and even laser pointers. It is a fundamental principle in laser physics and is used in various applications, such as laser therapy and laser ranging.

5. How can the inverse square law be used to calculate laser intensity?

The inverse square law can be used to calculate laser intensity by using the formula I = P/(4πr^2), where I is the intensity, P is the power of the laser, and r is the distance from the laser source. This formula shows that as the distance increases, the intensity decreases, and vice versa.

Similar threads

Replies
3
Views
1K
Replies
7
Views
3K
Replies
8
Views
4K
  • Mechanics
Replies
5
Views
1K
  • Optics
Replies
6
Views
1K
Back
Top