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Lasers, photo-electric effects and ionization of atoms

  1. Mar 31, 2008 #1
    I get the photo-electric effect but I am just wandering, is that the only principal on wich lasers function? I mean do lasers "cut" materials by ionizing the atoms?
     
  2. jcsd
  3. Mar 31, 2008 #2

    dst

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    Lasers typically cut metals by heating them up to melting point. When applied, they typically vapourize metal at just that region, and heat is dissipated very fast which is why they're used - to give very decent cuts where CNC milling wouldn't be the greatest option.

    No need for ionisation, just pure energy transfer. Also, I don't think the photoelectric effect is the most appropriate way to describe light interacting with solids. I don't know what is though, but plenty on here do.
     
  4. Mar 31, 2008 #3
    What you say makes sense, however if lasers did cut metal by heating it to melting point, I would think that this happens by exciting the electrons in the metal, wich would lead to more kinetic energy and thus increase heat. If this is true than why do mirrors reflect lasers beams, afterall the same that happens to the metal's atoms should happen to the mirror's atoms?
     
  5. Mar 31, 2008 #4

    dst

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    It depends vastly on the wavelength. Speaking from a generalised point of view, every material has a characteristic absorption spectrum - for each wavelength of electromagnetic radiation, we can say it'll absorb x% of it. For metal-cutting, you would use a laser tuned to maximum absorption (and hence maximum heating) which I think is in the infra-red range. It doesn't matter if it's transparent to us or reflective to us as long as absorption takes place in whatever range of wavelengths that is viable.

    I have no idea why reflection occurs to tell you the truth. It doesn't seem to be actually explained anywhere (at least pre-undergraduate level).
     
  6. Mar 31, 2008 #5
    Then we can assume that air (oxygen, nitrogen ect.) have practically no absorption, since the whole electro-magnetic spectrum can travel through air. This means that it would be impossible to cut solid oxygen with any type of laser, right? Do you know if a mirror can reflect X-rays?
     
  7. Mar 31, 2008 #6

    Integral

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    Fact is high power laser systems do not use metalized mirrors. They use what is called a dielectric mirror, it consists of multiple layers of a dielectric material, the thickness of a layer is determined by the wavelength of the laser and the angle of reflection needed. These mirrors are transparent to all but the wavelength designed for.
     
  8. Mar 31, 2008 #7

    dst

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    It depends on the absorption spectrum of oxygen. I would imagine that solid oxygen has a slightly different spectrum from gaseous oxygen (because these things mostly come from the bulk properties of a material and not the individual atoms).

    As for x-ray reflecting mirrors, again, I have no idea. Considering that mirrors are simply just a very convenient way to execute a quantum mechanical trick, then surely an x-ray reflector should be possible. In fact there is research going on into antennas (the sort that you use to pick up TV and radio) being used to pick up and transmit visible light signals. This of course is not the same thing as an x-ray reflector but EM waves are just that, there is nothing special about any particular wavelength or range of wavelengths, other than the arbitrary significance we associate with them.

    As I said, I honestly have zero idea as to why reflection occurs! Here's what Wikipedia says on the matter:

    In fact I did read the book cited and came out knowing less :rofl:.


    Oh, I might add - air has very little absorption mostly because it's far less dense than a solid. In my early education, that corresponds to a very very low decay/absorption constant (look up the exponential law of absorption). Whereas, with a solid there is a much higher chance of absorption.
     
    Last edited: Mar 31, 2008
  9. Mar 31, 2008 #8
    Thank you.
    I have another question though. Diffusion occurs when a wave enters a optical denser material or visa versa, for instance, a light wave moving from air into water will change direction. Does the light wave's direction change at the first water molecule that reemits it or does it happen deeper into the water, and why doesn't the direction of the wave change again as it is reemited from the second molecule?
     
  10. Mar 31, 2008 #9

    dst

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    I think he was talking about mirrors outside lasers :). But hey, I didn't actually know that snippet.


    That's refraction, not diffusion. I think it's covered in one of the FAQ threads here. Other than that, I honestly have no idea (I'm actually trying to find out why myself).
     
  11. Mar 31, 2008 #10

    Integral

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    So was I.
    Here is what wiki has to say
     
    Last edited: Mar 31, 2008
  12. Apr 3, 2008 #11
    Sorry, I meant refraction. It's a translation error on my part.
     
  13. Apr 16, 2008 #12
    The whole electromagnetic spectrum is really really large (really infinite)

    At any rate, both molecular nitrogen N2 and molecular oxygen O2 absorb very strongly in the ultraviolet. Almost 100% of the incident light from the sun below 190 nm is absorbed. Modtran is a wonderful program for simulating transmittance through the atmosphere

    google Modtran and Hitran
     
  14. Apr 17, 2008 #13

    Redbelly98

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    x-rays can be reflected at large (close to 90 degrees) angles of incidence. This principle has been used by astronomers to make x-ray telescopes.

    See http://en.wikipedia.org/wiki/Wolter_telescope
     
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