Laser light shows the unique property of coherence. This is due to the stimulated emission process of photons. But why is it that stimulated emission leads to coherence?

  • $\begingroup$ Because the stimulated photon is coherent with the stimulating photon, a process that can continue through the gain medium. $\endgroup$
    – Jon Custer
    Jun 5, 2020 at 13:30
  • $\begingroup$ Exactly ,in the gain medium when the stimulated photon passes another excited atom , it emits a photon in the same direction which is coherent with the latter. My question is why does this happen? $\endgroup$ Jun 5, 2020 at 13:42
  • $\begingroup$ @JonCuster Isn't your comment equivalent to "It is coherent because it's coherent"? I mean, the fact that the stimulated photon is coherent with the stimulating photon is stipulated in the question, no? $\endgroup$
    – garyp
    Jun 5, 2020 at 14:56

1 Answer 1


In this link stimulated emission is explained in the third page:


Note that the energy of the incoming photon doing the stimulation is $E_2-E_1$ the two energy levels . This means the first photon passes and a second one comes out, which when conditions are appropriate can lead to the lazing phenomenon.

This is with pictures and words, but coherence is a mathematical phenomenon. It means that one has solved the quantum mechanical problem and wavefunctions exist which have a fixed phase between them, the definition of coherence:

In physics, two wave sources are perfectly coherent if they have a constant phase difference and the same frequency, and the same waveform. Coherence is an ideal property of waves that enables stationary (i.e. temporally and spatially constant) interference.

Now you ask:

But why is it that stimulated emission leads to coherence?

Because of the mathematical solution of the quantum mechanical wave equations of the problem, it comes out in this situation that the waves are coherent. One has to dive into the mathematics if really wanting to understand how it happens.

  • $\begingroup$ As an aside on coherence :A classical example of coherence is that soldiers crossing a stone bridge have to break step, if in step they are coherent and the addition of the steps could resonate with the bridge and break it. $\endgroup$
    – anna v
    Jun 5, 2020 at 14:16
  • $\begingroup$ Exactly. It’s resonance that is the source of the coherence. $\endgroup$ Jun 5, 2020 at 14:25
  • $\begingroup$ I think this answer is backwards. You say that the phenomenon of coherence arises from the mathematics of quantum mechanics. I think the mathematics is invented to describe the phenomenon of coherence. $\endgroup$
    – garyp
    Jun 5, 2020 at 15:00
  • $\begingroup$ @garyp sure, but without the mathematical model there is no explanation other then : "this is the way things are". In addition mathematics is predictive, which validates the model $\endgroup$
    – anna v
    Jun 5, 2020 at 15:06

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