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Light beam question

  1. Aug 10, 2005 #1
    I was wondering somewhat randomly, and pointlessly about the nature of light beams and how ordinary light sources emit beams in all directions in a very un-concentrated manner. And of course lasers emit a beam that almost totally focused on a single point. So I was wondering, would it be possible to say, place a lightbulb inside a system of mirrors that redirected and refined the light to a single point that would be usefull for long-range transmissions? Say several miles. In other words could you make a system of mirrors that could re-focus all random beams of light into a single, laser-like powerfull beam.

    Thanks for the help! :)
  2. jcsd
  3. Aug 10, 2005 #2

    Claude Bile

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    Laser beams have several qualities that differentiate them from other light sources.

    - They are highly directional (Power flow is confined to a small angular range).
    - They are coherent (All the photons are 'in step' with one another).
    - They are monochromatic (Mostly, there are some lasers that operate over a broad spectrum, so called supercontinuum lasers).

    A light source must have all these properties to imitate a laser beam in terms of power output.

    Even if you had a perfect point source and had an elliptical cavity to focus the light, there is no way to guarantee all the photons will be 'in-step' for two reasons. One, incoherent sources typically emit over a broad range of wavelengths, different wavelengths quickly move out of step as they propagate. Two, the mechanism by which this radiation is emitted does not guarantee that the photons will be in phase at the point of emission.

    So, in summary, no, it is not possible to turn an incoherent source into a coherent source using mirrors.

  4. Aug 10, 2005 #3
    Thanks Claude.

    Ok so it couldnt imitate a laser. But it could refocuse the light to a single, very powerfull point. My main question is, and I should have state this more clearly, how long (distance) could these photons hold that "focus". Naturally, over distance the photons would spread out, but by how much? Is it possible that it could retain most of its focus over perhaps miles?

    Thanks again :)
  5. Aug 10, 2005 #4
    does the word collimation says anything to you?

    Best Regards,

  6. Aug 11, 2005 #5


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    There are several physical limits that apply.

    The major one is probably this: with imaging optics, you cannot increase the source brightness (the power density per unit solid angle).

    There are also some diffraction limits, but if you have large lenses they won't matter to your problem.

    You could certainly build a lens to focus your light over a distance of miles. I don't recall offhand how you calculate the spot size from the geometry, though. Google finds:


    which should probably tell you more than you want to know :-)
  7. Aug 11, 2005 #6


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    A parabolic mirror on a spotlight does what you are describing.
  8. Aug 11, 2005 #7
    This is going to sound stupid: what do you mean when you say a laser has power?

  9. Aug 11, 2005 #8

    Claude Bile

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    Jake, When focusing anything, you basically have a trade off between resolution and depth of field (how long it holds focus, otherwise known as the Rayleigh range of the beam). The shorter the focal length of your objective, the smaller the focus will be, however the Rayleigh Range will also be reduced. Longer focal lengths will not be able to focus the beam down as far, however the light will be focused over a much longer length (not miles, you are probably looking at distances more like cm, or at the very most a metre or so).

    Kitty, Power is a quantity with units of Watts, or Joules per second. When we say a laser has a power of 10 W, we mean that it is outputting 10 Joules of energy per second. Lots of lasers however are not continuous wave (CW), but pulsed, so one must differentiate between averaged power and peak power. A femtosecond pulse Ti:Sapphire laser may have average powers of milliwats, but peak powers can easily go into the Gigawatts (billions of watts) thus making them much more dangerous than a CW laser with the same average power.

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