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Help Newbie

  1. Apr 4, 2006 #1
    First of all excuse me if i'm posting in the wrong section or you are simply annoyed by my questions.
    It just happens that i love all this scientific stuff and althoughi don't always understand everything i try to.

    I just finished reading a very basic article about a laser operates and it talked about the excitement of a sort of particle which when struck by a photon and already excited would emit 2 photons and in the same direction that the first photon came.

    It also mentioned something about starting a sort of chain reaction to get all the "particles" excited. The ilustrations showed two mirrors but one of them being only partially reflective. This caused most photons to bounce back and forth exciting the particles and "creating" more photons. Some of this photons did escape the "device" trough the partially reflective mirror thus creating a laser beam.

    Now comes my stupid question:

    What if instead of a partially reflective mirror in one end i used both mirrors completely reflective?? Wouldn't it escalate whatever few photons i dumped in it infinitely????

    And to help me understand even if my question is completely idiotic, i'd like to know what kind of "particles" are used that cause that effect of duplicating photons when already excited and struck a second time by another photon.

    Thanks for your patience and understanding with a simple man that seeks a little chunk of wisdom.

  2. jcsd
  3. Apr 4, 2006 #2


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    Welcome to PF,

    It is impossible to get something to totally reflect all light, but hypothetically you would get a large source of concentrated energy and i dare say a pretty spectacular explosion. The process by which other photons are created is called stimulated emission. Basically the original photon interacts with the excited electron of the atom and it emits its energy as a photon with the same properties as the original photon. Standard gases in lasers are helium and neon and also argon, hydrogen fluorine and many many more depending on what wavelength of light you want.
  4. Apr 5, 2006 #3


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    As Kurdt already said, the excited particles are simply atoms or molecules (or "pseudo-electron levels" in a solid). Apart from the difficulty of making a mirror with no losses, what would in fact happen is this:
    there must be an energy source that "pumps" the atoms into their excited state. There are different ways to do so ; it is the reason why a laser consumes electricity ! This "pump" can only excite so much atoms per second. Now, when a photon meets an excited atom, it can undergo a process called stimulated emission: one photon in, two photons out (so to speak). But when that same photon meets a non-exited atom, it can get absorbed and excite the atom (one photon in, zero photons out). One must also know that atoms can deexcite spontaneously.
    So we have a "constant production rate" (by the pump) of excited atoms, and then a "growing beam" that "consumes" excited atoms to grow, on one hand, and "produces" excited atoms (but diminishes the beam) on the other hand ; and finally a "leakage" of spontaneously deexciting atoms.
    All this will come to an equilibrium when the "production" equals the losses. At that point, the beam will induce as much stimulated emission as it will induce absorption (so that its average intensity will remain steady), and the populations of excited and unexited atoms will remain stable (as many go in one direction, as there are that go in the other direction).
    The pump will replenish exactly those excited atoms that got de-excited through other means than interaction with the beam.
  5. Apr 7, 2006 #4
    So my mistake was to think that "new" photons where coming out of nowhere when an excited atom was hit by another photon and two came out because actually, it took one photon to excite that atom previously right???

    Anyway, another stupid question:

    I now understood that the laser needs to be fed at all times by a source of electricity that keeps the atoms excited. What if i could use two reflective surfaces instead of one of them beign semi reflective to accumulate energy inside the "system" and then release it???
    Would it be a sort of pulse laser or just another idiotic rambling???

    Thank you all for your help and please remember i know very little about this so don't get mad if i make horrible mistakes :)
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