Why do we see streaks of light?

Factual Questions
21

Zombie Light Thread. Wow.
If the OP is referring to the “streaks” of light you see around bright light sources at night, then these are definitely due to diffraction. The diffraction comes from the aperture the light passes through, in this case, your iris at the edges of your pupil.

Although Jearl D. Walker’s Flying Circus of Physics ascribes these diffraction effects to irregularities in the edge of the pupil, the sources he cites don’t actually back him up in this. as far as I know, no one has performed the definitive measurements to establish this – but see below.

the iris of your eye expands and contracts, and is not a perfect circle. Practically no system that changes size is really a perfect circle. Camera irises are sort of polygons with curved sides. I think that the it is is actually more like the end of a draw-string bag, with the extra material gathered into “lumps” at the edge rather than sliding over each other like the vanes in a camera iris.

From the point of view of diffraction, it doesn’t matter. If the iris were a perfect circle, you’d see perfectly round halo-like diffraction rings around each light source.

But with a segmented, quasi-polygonal opening, you see “rays” coming out from the center. Here are diffraction patterns from regular polygons:

https://www.google.com/search?q=diffraction+from+polygonal+aperture&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiZy4mvo7fhAhUliOAKHfX4B2AQ_AUIDigB&biw=1440&bih=708#imgrc=ySoJlXacre4azM:&spf=1554409737146

https://www.itp.uni-hannover.de/fileadmin/arbeitsgruppen/zawischa/static_html/stars.html

Your iris isn’t a regular polygon with relatively few sides, though – it’s a very irregular polygon with LOTS of sides, so you see a profusion of lots of rays.
There has actually been a lot of discussion about exactly what structure is responsible for the “rays” effect, but I was able to prove to myself that it was definitely due to the irregular edges of my iris. Since I work in optics, it was relativelty easy for me to find a largish “pinhole” – a very regular hole cut into a thin piece of metal. The one I used was about a millimeter in diameter. at that size, it’s smaller than the typical diameter of my iris, especially in dim light (when it opens up). Holding the pinhole in front of my eye and looking through it at night toward a distant bright source, I no longer saw those characteristic “rays”, but saw the halo-like Airy rings around a central round spot. That wouldn’t have been the case if the rays had been the result of striaeae in the lens of my eye, or structures in my cornea, or even because of my eyelashes.

22

I had coma aberration in one eye due to scar tissue distorting the cornea. This caused point light sources to have smeared-out tails, like the tails of comets (hence the name). For a long time I thought it was due to streaks on my glasses or windshield, but the streak always pointed in the same direction relative to the orientation of my head; that’s when I realized it was a problem with my eye.

23

Excellent thinking. I can tell which aberrations are present in my eyes from the appearance of light streaks, too. In fact, you can usually tell which ones are present in each eye by looking with one eye at a time at the distant source.

24

Isn’t this just what people see when they have astigmatism? Like that recent viral tweet about streaking lights at night: Astigmatism lights comparison: It likely won't diagnose the condition The (banned)OP already ruled out cataracts, and if he’s driving we can presume he’s had eyetests that would detect short-sightedness, though those should detect astigmatism too which leaves me baffled about why so many people responding to that tweet didn’t know they had it.

25

I have astigmatism, and I see nothing like those streaks in that viral tweet. I just took my glasses off and looked at various points of bright light outside, and no streaks whatsoever. Just an indistinct glow instead of the sharp object I see with my glasses off. I didn’t see them seven years ago and I still don’t see them today.