Why does a laser pointer have a grainy pattern in the beam?

The light from a laser is coherent, all the waves of the same wavelength, color, and all in exact lockstep with one another. When the beam strikes a surface and bounces off and the wavefronts collide with one another and can reinforce and/or cancel one another.

The sparkle pattern of a laser can be compared to the choppy surface of a lake on a windy day as waves cancel and reinforce one another to the extent that no regular pattern of waves can be seen.

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