Context: https://www.reddit.com/r/askscience/comments/uruuh8/if_spaceships_actually_shot_lasers_in_space/?utm_medium=android_app&utm_source=share
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There are many ways to approach it.
The simplest is probably [Huygens–Fresnel principle](https://en.wikipedia.org/wiki/Huygens%E2%80%93Fresnel_principle). In this model any light wave can be thought of a sum of infinitely many point light sources. The sum of these sources if same as the wave you want to model.
Now if you have infinitely wide plane wave you can have these point sources on its infinite width and everything is nice. The plane wave will stay same and propagate to infinity without spreading.
But our wave sources rarely are infinitely wide. You have some sort of aperture.
Image: https://en.wikipedia.org/wiki/File:Refraction_on_an_aperture_-_Huygens-Fresnel_principle.svg
Now the point wave sources are only on the width of the aperture. If you sum out the waves from the point sources you will see that the wave is no longer neat plane wave. Instead it spreads out.
How exactly it spreads out depends on the shape of waves and shape(s) of the aperture(s). But no matter how you choose the waves the light will always spread out after passing through an aperture.
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