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Simple question, I've always wanted to know the answer to this.

Why do you see a pair of lines radiating out from street lights when your squint at them? I can't think of a better way of describing what I mean, but I hope one know what I'm talking about.

Is it some sort of diffraction effect causes by your eyelashes acting as a grating?

Any ideas would be greatly appreciated :-)

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Diffraction seems plausible. - This random discussion forum brings up another possibility: The eyelids distort the shape of the liquid on the surface of the eye, causing bad lensing in the vertical direction. That one seems plausible too. I don't know which (if either) is correct. – Steve B Aug 15 '12 at 13:41
I'm not sure everybody sees what you see. After I had Lasik surgery, I saw an irregular polygon with wiggly sides around each point source light. I attributed this to my pupil being open wider than the corrected part of my lens: so the inner part of my lens was correct and focused light to point and outside part of my lens was uncorrected by lasik and focussed light to a blur and I saw these overlapping. – mwengler Aug 15 '12 at 19:01
Although this might not be connected, after switching from glasses to contact lenses all street lights possess a circular halo, with accompanying lines that stretch from the centre of the lamplight to the edge of the circle. – Ari Ben Canaan Dec 7 '14 at 5:15

2 Answers 2

This is apparently a diffraction pattern due to eyelashes and perhaps even eyelids; closely related to "lens flare", "diffraction spikes" or "aperture stars".

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This is an informative video, which also relates to the by you mentioned phenomena. However I think it is only/mainly due to the diffraction of the eyelids, since the diffraction pattern would have to be perpendicular to opening. – fibonatic Dec 7 '14 at 3:53

A few simple observations will clarify what you see.

If the lines are mostly vertical, the are not caused by your eyelashes (which would create a horizontal diffraction pattern).

Vertical lines might be caused by diffraction if you can close your eye almost completely - the width of the aperture has to be a few wavelengths at most to see any diffraction "lines". If this is the mechanism then you would expect to see repeating fringes for monochromatic lights (like the yellow sodium street lights), and colored fringes in the vicinity of the center of the light.

More likely, surface tension will cause the liquid on the surface of your eye to "bead" causing a cylindrical lens. This will give a strong vertical distortion perpendicular to the opening between your eye lids. Your two eyes have slightly different angles and this would give rise to two different lines in your stereoscopic view of the world.

A better description of what you see would help!

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