1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why does a (laser) light beam not disperse?

  1. Apr 16, 2008 #1
    Does anyone know where I can find a derivation how/why a light beam disperses? I'd like to see some sort of solution to Maxwell's equations for a finite width EM wave in free space.

    If the beam is a linear superposition of frequencies and Maxwell's equations are linear, does a laser beam disperse less than an equivalent beam of normal light?
     
  2. jcsd
  3. Apr 16, 2008 #2

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    First, a laser beam does disperse, just not as much as "regular" lights. Laser beams are very closely 'collimated', much like a search light so it doesn't disperse very much to begin with. But the key to laser light is that it is NOT a "linear superposition of frequencies". The difference between laser light and "regular" light is that the laser's light is restricted to a very short range of frequencies (the ideal would be one frequency but that, strictly speaking, is not possible).
     
  4. Apr 16, 2008 #3
    So can laser light be described by a classical EM wave of a single frequency or do you require quantum mechanics?
    I'd like to calculate this in detail with actually solutions to Maxwell's equation. From undergrad studies I know calculations for waves in free space and wave guides. Not sure what's the easiest way to consider a finite width beam in free space. Any references?

    Why should a mixture of frequencies disperse more than each of them individually?
     
  5. Apr 16, 2008 #4

    Andy Resnick

    User Avatar
    Science Advisor
    Education Advisor
    2016 Award

    The lowest mode laser beam is well described as a Gaussian. A google search for "gaussian beam propogation" or "abcd matrix" should pull up something useful.

    Laser light is usually well described classically.
     
  6. Apr 17, 2008 #5

    Redbelly98

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Just a question, aren't we really talking about diffraction in this thread? Dispersion is what results from a variation in speed (or equivalently, refractive index) with wavelength.
     
  7. Apr 18, 2008 #6

    Andy Resnick

    User Avatar
    Science Advisor
    Education Advisor
    2016 Award

    Good catch- yes, diffraction and dispersion are entirely different.

    Laser light, being nearly monochromatic, displays significantly less dispersion in optical systems than braodband radiation.
     
  8. Apr 19, 2008 #7
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Why does a (laser) light beam not disperse?
  1. Laser Light (Replies: 14)

Loading...