Diffraction of a Laser

  1. I've never understood how diffraction is possible, which has led me to mistrust a good deal of quantum physics. I'm mystified as to how light starts spreading out in all directions after passing through an aperture. Supposedly it's because light spreads out like a wave, but then how is it possible to have a laser that spreads out very slowly? And why does the laser start spreading out much faster after passing through an aperture? I've never understood diffraction because I've never gotten an answer to this question. Perhaps someone here can enlighten me.
     
  2. jcsd
  3. I think you need to understand that it is not just light that diffracts, all forms of EM wave can be diffracted.

    The amount of diffraction depends on the wavelength of the wave; so the fringe spacing of diffracted light can be solved by: [tex]w = \frac{\lambda D}{s}[/tex] where [tex]\lambda[/tex] is the wavelength of light incident to the grating of size [tex]s[/tex] and distance [tex]D[/tex] from the diffraction pattern.

    We can see from this equation that as the wavelength of incident light decreases, so does the space between maxima; which is what I understand by our use of 'spreads out very slowly'

    Hope this helps,

    C
     
  4. Andy Resnick

    Andy Resnick 5,943
    Science Advisor
    Education Advisor

    That's a good question- a surprisingly good question!

    The 'raw' laser beam does indeed spread out 'more slowly' than if you were to send it through a pinhole aperture. But the reason has to do with how the laser beam is generated inside the lasing cavity as well as diffraction through an aperture.

    Inside the laser, the resonant cavity has a certain shape- not only length, but width as well. As the light builds up (the exit window only lets a small fraction of light out), it develops not only longitudinal modes (the wavelength) but also transverse modes (so-called Gaussian, Hermite, etc.). Typically, a laser exits in the (0,0) TEM Gaussian mode. These modes are carefully matched to the size of the exit window (or vice-versa, the exit window diameter is specified by the beam waist of the desired beam profile). The light exits with a divergence (spreading) inversely related to the exit pupil size. ([tex]\theta * \omega_{0} = \pi\lambda[/tex])

    http://en.wikipedia.org/wiki/Gaussian_beam

    For a green laser, the beam divergence can be milliradians fairly easily.

    Now, what's the difference between that and passing the light through a hole? If the hole is the size of the beam, then nothing! If the hole is smaller than the beam, there are *additional* diffractive effects from truncating the beam- and the more truncation occurs, the 'faster' the beam diffracts out.
     
  5. Andy Resnick's post pretty much nails it, but I would like to ask the OP a question, if I may.

    First of all, why has light diffraction caused you to distrust QM? Light diffraction happens, it's a fact, and you can observe it by doing the experiment yourself. It's a fact of nature. A physical theory is, at best, just an attempt to explain or model natural phenomena.

    Light diffraction isn't even QM-specific. Classical electrodynamics predicts diffraction too.

    Also, here is a similar thread that you might want to read (Some people answered this question nicely): https://www.physicsforums.com/showthread.php?t=376754
     
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