Why does laser dot become bigger at bigger distance? e.g: from millimeter to inches so that it distracts pilots in aircrafts. I knew that laser beam should't expand over distance, but remain stable. Is that true?
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5Laser beams diverge like all light does. That's a consequence of the wave nature of light. – CuriousOne Sep 15 '15 at 06:25
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3possible duplicate of Why does a laser beam diverge? – Sep 15 '15 at 11:19
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This problem can be answered if we treat Light as a Particle. Due to Heisenberg's Uncertainty Principle, $\Delta x\Delta p\geq \dfrac{\hbar}{2}$, saying that we cannot exactly know the particle's Position and Momentum at the same time. In the case of a laser, a photon cannot have 0 momentum in any arbitrary direction so the photons cannot just go in one direction together. So what little momentum in other directions will cause the photons to spread out over time.
Brendan Lim
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This isn't right, but it has a grain of truth to it. The divergence is a fundamental property of solution of Helmholtz's equation: a finite beamwidth means that the superposition of plane waves needed to realize the finite beamwidth spreads in direction more and more as the beamwidth decreases. It is mathematically analogous to the HUP because the spread in directions of the plane waves is the spread in Fourier (momentum) space of the beam, and this is related to the spread in untransformed space by the Heisenberg inequality. Light of course does have an HUP, but there is no position.... – Selene Routley Sep 15 '15 at 10:47
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... operator for the light field. The conjugate variables governed by the HUP for light are e.g. the electric and magnetic field observables. Have a look at this answer of mine, with diagrams http://physics.stackexchange.com/a/79609/26076 explaining the divergence more. – Selene Routley Sep 15 '15 at 10:57