How come when you squint at a bright light, light beams seem to appear on the top and bottom of the source?
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What you are seeing is the diffraction of light through your eyelashes. As light passes by the barrier of your lashes the light is bent and the sum total of this interaction is the light “beams” you describe, actually called “diffraction spikes”.
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No Reddit! Not again! Someone posted a photoshopped picture of a streetlight with diffraction spikes and people were commenting about how cool it looked and how it looked like looking at lights through a rain covered window. I was sitting there in total confusion, because ALL lights look like that to me! It can be a television or a fish tank bulb, all light sources have the spikes going in all directions. I always assumed that everyone saw light like that. Well Reddit thanks for ruining my idea of me having perfect vision.
Normally, your tear film forms a uniform layer over the front surface of your eye.
When you squint, your eyelids act as a wiper blade to sweep up a “wave” of tears from the top and bottom eyelids. This tear wave is curved vertically and acts as an cylindrical lens to spread out light into your eye in the vertical direction.
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