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 you squint at them? I can't think of a better way to describe what I mean, but I hope someone knows what I'm talking about.

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

Any ideas would be greatly appreciated :-)

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    $\begingroup$ Diffraction seems plausible. boards.straightdope.com/sdmb/showthread.php?t=630308 - 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. $\endgroup$ – Steve Byrnes Aug 15 '12 at 13:41
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    $\begingroup$ 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. $\endgroup$ – mwengler Aug 15 '12 at 19:01
  • $\begingroup$ 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. $\endgroup$ – Ari Ben Canaan Dec 7 '14 at 5:15
  • $\begingroup$ I see lines without squinting! $\endgroup$ – Santropedro Nov 20 '17 at 23:23

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

  • $\begingroup$ 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. $\endgroup$ – fibonatic Dec 7 '14 at 3:53
  • $\begingroup$ I don't think this is correct. A diffraction pattern would separate colors, but I've never observed this. $\endgroup$ – knzhou Jan 12 at 21:34
  • $\begingroup$ @knzhou: "A diffraction pattern would separate colors" -- Indeed, cmp. en.wikipedia.org/wiki/Diffraction#/media/… "but I've never observed this." -- Obviously this would require the light source to have some spectral range, and you to have sufficient sensitivity ... (While trying to research more "practical estimates" I just now came along this, btw. ...) $\endgroup$ – user12262 Jan 31 at 21:12
  • $\begingroup$ Yes there is colour dispersion though you can ask your eye/eyelids/eyelashes to act as monochromator. I am here just because yesterday I watched at a harbour and basically got a laser show! I do have problems on drawing a scheme. It must be diffraction from the eyelashes edge, isn't? $\endgroup$ – Alchimista Feb 14 at 9:51

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!

  • $\begingroup$ Yesterday I saw this in a spectacular way (many lights giving what I might described as a laser show). Can you explain why eyelashes should not give a vertical line?* I had to say here that the line for far away sources was splitted in quasi parallel lines. Might be eyelashes shadow. ... Thanks. In addition there seems to be a Maddox effect due the liquid film on cornea. Indeed I was a bit crying due to air. *as the width of a grate gives vertical fringe shouldn't be the eyelashes seen as a "negative" of it? $\endgroup$ – Alchimista Feb 14 at 10:27

You can test if this is your eyelashes and also see an optical illusion of sorts. First, when you see these lines, open your eyes wide and notice if they go away or not. In my experience this works with virtually any relatively dim light in a dark space. A candle in a dark room from far enough away or a street light that is far away. If the lines of light go away when your eyes are opened wide then the lines of light were light reflecting off of your eyebrows and into your pupil. It is essentially the same thing as being able to see your eyelashes as blurry edges when you squint in a more well lit space. To see the optical illusion, close your eyes in the right circumstances, enough to make the lines of light appear. Cover your bottom eyelashes with your index finger carefully, without disturbing your top eyelash. The lines of light at the top of your vision will disappear when your finger is covering your bottom eyelashes. To make the lines of light at the bottom of your field of view disappear, cover your top eyelashes.