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It is not correct to say that "the waves in laser light are all in phase." When two light waves combine, they inextricably add together to form a new wave, they do not travel as two independent "in-phase" waves. The photons in laser light are in phase, but the WAVES are not. Instead, ideal laser light acts like a single, perfect wave.
When the light wave within a laser causes atoms to emit smaller, in- phase light waves, the result is not "in phase" light. Instead the result is a single, more intense, amplified wave of light. In-phase emission leads to amplification. If the atoms' emissions weren't in phase, then the atoms would absorb light rather than amplifying it.
Each atom in a laser contributes a tiny bit of light, but their light vanishes into the main traveling wave. The light from each atom strengthens the main beam, but loses its individuality in the process. 99 plus 1 equals 100, but if someone gives us 100, we cannot know if it is made from 99 plus 1, or 98 plus 2, or 50 plus 50.
All the *PHOTONS* in a single wave of light are in phase. This might be one reason that people say that laser light is "in phase" light. However, in-phase photons are nothing unique, and they don't really explain coherence. For example, all the photons radiated from a radio broadcast antenna are also in phase, but we don't say that these are special "in phase" radio waves. Even if all the photons in laser light are in phase, it is still incorrect to say "all the waves are in phase." Photons are
not waves. They are quanta, they are particles, and they do not behave as small, individual waves.
The light from a laser is basically a single, very powerful light wave. Single waves are always in phase with themselves, but it's misleading to imply that a single wave is something called an "in phase" wave. Laser light could more accurately be called "pointsource" light. The physics term for this is "spatially coherent" light. Light from light bulbs, flames, the sun, etc., is called "extended-source" light. Starlight and the light from arc welders is "pointsource" light and is quite similar to laser light. Light from arc-welders and from distant stars has a higher spatial coherence than light from most everyday light sources. (Note: the sun is a star, correctly implying that light becomes more and more spatially coherent as it moves far from its source. This is a clue as to the REAL reason that lasers give spatially coherent light!
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