laser principle


laser principle for other animations and explanations about quantum physics
realisation Data-Burger, scientific advisor: J. Bobroff, with the support of : Univ. Paris Sud, SFP, Triangle de la Physique, PALM, Sciences à l’Ecole, ICAM-I2CAM



  1. why don't the atoms, once losing their excitement and reaching a lower energy level then absorb one of the light rays and become excited again?

  2. Various wrong beliefs most students have acquired about lasers:

    * light is shaped like a sine-wave in space! (Nope, wrong, that's the obsolete "transverse Aether-vibration" viewpoint. How wide is the side-to-side movement? Zero, there is no such movement. Think: brighter light isn't "wider.")
    * individual sine waves can "pack together" side by side. ( No, doesn't happen. Light doesn't have those sine-shapes, nor sideways motion. Also, in basic wave-physics, smaller waves vanish as they add together, where two waves become a single larger wave.)
    * laser light builds up, then it suddenly breaks through the mirror at the end! (No, there is no such burst. Some light leaks out during every single reflection, also building up at the same time.)

    Unfortunately the above video reinforces all these common misconceptions. And, perhaps students who didn't suffer these three mental errors …will acquire them from this video!

    It's possible to construct animations which don't produce or reinforce common physics-misconceptions. Unfortunately, nobody ever tries this. Perhaps the authors really believe that photons wiggle sideways? Or, perhaps this is "Stephen Gould Creeping Fox Terrier" effect, where authors can only copy earlier textbooks/animations, rather than creating vastly improved original versions from scratch.

  3. Thank you very much, very helpful!
    I have 2 questions:
    1) How to make atoms excited?
    2) And how it is possible to make 'Red' coloured laser or other colours?

  4. Edit: Nevermind, what I've described is spontaneous emission, like in LEDs. The video is right. The initial excitation is caused by the voltage, and initially emission is spontaneous but these emissions will hit other excited electrons, resulting in stimulated emission. Voltage is then reduced to maintain a stable level of emission. Original post below.

    I can't help but feel like this explanation is wrong.. 

    My understanding is that the initial emission is the result of voltage being applied to the semiconductor, that is, electrons and holes recombine across the depletion region and emit light. This light is then reflected around and excites further electrons, which recombine and emit light themselves (as in the video, except the light excites the electron, the light is consumed, THEN the electron returns to the lower state by recombination and emits a single photon). The voltage applied is regulated to increase the amount of light inside until such a point that the device emits enough light to sustain itself, at which point the voltage is reduced to maintain that level of light emission.


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