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