why does the point of a laser look pixelated when spread out?
In: Physics
The thing about laser light that makes it special is that it’s ‘coherent’, which means it’s all coming out of the emitter at the same frequency and phase. But once it leaves the emitting element, it passes through some stuff. First, usually, a little bit of transparent plastic which is part of the laser diode package. Then, possibly, a bit of glass or plastic optics meant to bend or focus the light. Then some air. Each of these transitions bends the light beam a little. This means that different photons might be leaving the same spot on the emitter, and landing on the same spot on the wall (or whatever) but they travelled along different paths in between.
If those paths are not exactly the same length, then that means the photons are no longer guaranteed to be in phase with each other. If they are in phase, then they interfere constructively and are brightly visible. If they are out of phase, then they interfere destructively and will cancel each other out.
[a little diagram of how constructive and destructive interference works](https://cdn.britannica.com/07/62907-004-A3A6351E.jpg)
The speckled, mottled pattern you see around a laser pointer dot, is the result of those photons following slightly different paths and messily interfering with one another.
That principle – light from a single source travelling different paths and interfering with itself – is basically the same phenomenon they were exploiting in the [Michelson-Morley experiment,](https://simple.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment) which demonstrated some important facts about the speed of light and helped to confirm Einstein’s theory of relativity.
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