Physics Help Forum basic action of a laser

 Oct 26th 2009, 12:21 PM #1 Senior Member   Join Date: Sep 2009 Location: india Posts: 409 basic action of a laser i faced some conceptual difficulties while studying lasers from my text. the text (HCV-indian students will know) defines laser as a process by which we get light beam which is highly coherent, highly monochromatic and almost parallel rays whose photons have exactly the same phase. well, in wave theory i read light as an electromagnetic wave, so i understand what is 'phase' but WHAT is meant by'phase' of a photon? i thought particle nature of light was introduced discarding wave nature of light. next, it describes the mechanism of stimulated emission. what i did not understand was why stimulated absorption does not interfere with stimulated emission? what is the use of metastable orbits in helium neon laser(i found nothing special) lastly, for pure inquisition, how do laser printers and CDs work?? please note- answering all queries is not necessary. Last edited by r.samanta; Oct 26th 2009 at 09:07 PM.
 Oct 28th 2009, 01:40 AM #2 Physics Team   Join Date: Feb 2009 Posts: 1,425 Actually i dont think the wave theory has been discarded. Some phenomena like interference need us to assume that light or e.m radiation is a wave and others like photoelectric effect need a particle type of treatment. I do not know of any parallel analogy for phase of a photon. The spin of the electron is something similar yet opposite in the sense, it is a quantum mechanical property which has no classical analogue. It is definitely not the electron spinning around an axis. The presence of the photon stimulates the presence of other photons i.e. emission (something akin to resonance) If it were to be absorption what should stimulate that should be the absence of photons dont you think? However with a larger no of phtons available the absorption should also increase becaused of increased probability. For the lasing action to take place, a population inversion needs to occur i.e. we need a very large fraction of atoms in the excited state after they have been "pumped". Normally the excited atoms emit and come to a lower state, i.e. they dont stay excited long enough for population inversion to occur. A metastable state has a comparitively large lifetime, i.e. atoms in this state tend to linger there longer which facilitates population inversion. So normally atoms which have such states are used.
 Oct 28th 2009, 02:08 AM #3 Senior Member   Join Date: Sep 2009 Location: india Posts: 409 thank you ^(infinity)

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