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I came across this MCQ in my text book, it says:

The laser beam is a collimated parallel one, that means its photons have the same......

  1. direction 2. frequency 3. intensity 4. phase

I will not pick either frequency or intensity, but I am confused between phase and direction, could you help me? and explain for me which one is correct?

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  • $\begingroup$ Not sure I understand the question, but I'll try. Direction and phase are both correct. Don't know why you suspect otherwise. "Same direction" means all the photons are traveling in the same direction, which I think is clear. "Same phase" means that all the photons at any given distance along that direction all have the same vibrational phase as one another. $\endgroup$ Jun 7, 2017 at 21:34
  • $\begingroup$ And, strictly speaking, it is not a parallel beam, just a narrow divergence angle compared with other light sources. $\endgroup$
    – Jon Custer
    Jun 7, 2017 at 22:17
  • $\begingroup$ @bob.sacamento I am confused because that's a multiple choice question and I have to pick only one, and I think both direction and phase are good choices. $\endgroup$
    – Asmaa
    Jun 8, 2017 at 0:55
  • $\begingroup$ @JonCuster Ok, you are right. But what about my question? $\endgroup$
    – Asmaa
    Jun 8, 2017 at 0:56
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    $\begingroup$ Appears to be more of an English question than a physics one. Neither "collimated" nor "parallel" suggest anything about the phase. To me, the book's question is explicitly asking what the clause means, not necessarily what is true about laser beams. $\endgroup$
    – BowlOfRed
    Jun 8, 2017 at 4:26

3 Answers 3

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The photons emitted by the active medium remain for many round-trips in the optical cavity of the laser. Stimulated emission creates an avalanche of "twin" photons, which explains many of these laser properties. Specifically, collimation refers to photons having (roughly) the same direction.

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  • $\begingroup$ Ok, but in my multiple choice question I have to pick only one choice to describe the reason why laser is a collimated parallel beam. $\endgroup$
    – Asmaa
    Jun 8, 2017 at 1:04
  • $\begingroup$ @Asmaa See the edit $\endgroup$
    – scrx2
    Jun 8, 2017 at 10:24
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The dictionary definition of collimated:

collimated; collimating

transitive verb

: to make parallel collimate light rays

so the answer is single and one: They have the same direction.

The rest describe a laser beam, but not the meaning of "collimated". Phases can be fixed or random but not collimated.

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If they all had the same phase, then you'd have a static field at zero frequency.

Going along the trajectory of a plane-wave, each quarter-wavelength distance must change the phase by 90deg, and each half-wavelength distance changes the phase by 180deg. In other words, only a plane-wave with zero frequency and infinite wavelength could have the same phase everywhere.

Laser light is not "in phase." It's more accurate to call laser light "phase locked," with the phase-difference between different locations ideally being constant.

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  • $\begingroup$ Perhaps, but I think you are applying too much analysis to a very poorly worded question, one that has been simplified to the point of being physics nonsense. $\endgroup$
    – garyp
    Jun 8, 2017 at 11:34
  • $\begingroup$ @garyp perhaps, but the goal is the same with any test: by process of elimination to guess the answer intended by the author of the question. That may not work, given the author's insane beliefs about lasers and/or photons. (Heh, perhaps the "correct" answer is supposed to be phase? ) $\endgroup$
    – wbeaty
    Jun 9, 2017 at 5:02

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