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Physical Optics While Alone
Raindrops, Interference Fringes . . 1998 W. Beaty

> Mark Martin - 02:43pm Jun 11, 1998 PDT SAS FORUM
> I am interested in knowing if any serious research has ever been done or
> published on the optical streaks that one sees when squinting the eyes
> on a brilliant light source. It occurs to me what the underlying physics
> probably consists of, but this seems to be fairly complex in that the
> streaks are often directed in numerous angles simultaneously, perhaps
> having to do with the interference of the eyelashes. I'd certainly be
> grateful for any direction someone may be able to provide. Thanx.

Hey, I once used to mess with this all the time! Someone had asked me about it years before, and then I noticed that the phenomena was extremely bright and clear if I squinted at distant streetlights at night while waiting for the train after work. Often it was 30 minutes before the next train arrived, so I put in many hours at this pastime. It was legitimate work, I was doing Science Museum research for exhibit-design and answering everyday science questions. :) Don't stare at the sun, you'll go blind. Use streetlights in the night instead.

As P. Baum on SAS said, it is clear that the main vertical streak is caused by the short-FL concave cylinder-lens formed by the meniscus of tears along the edge of the eyelids. With teary eyes, the large amount of fluid creates big and wavery images, while with nearly dry eyes one can squint the eyes almost completely shut before the vertical stripe of light appears.

The edges of the vertical stripe are sharp and clear because the tiny cylinder lens does not affect light in the horizontal plane (at least not much.) A small source such as a distant streetlight will give a narrow vertical stripe.

Have you noticed that a horizontal "hump" of fluid can be formed on your eye's surface? View a distant streetlight at night, then cross your eyes hard in order to de-focus them. This causes the point-source streetlight to become a disk. (the disk is actually a shadow of your pupil, and if you shine a flashlight in your OTHER eye, you'll see the disk contract. Cats wouldn't see a circular disk!) There will be detrius appearing on the disk, shadows of stuff on your cornea surface. Blink a few times to "wipe the pane," and the specks and blobbies move around. If you now squint your eye almost closed, then open it wide and de-focus it to produce the disk, you'll see a horizontal band. This is the hump of fluid. Blink a couple of times quickly, and it goes away. If you view stars at night, occasionally this "hump" will cause every single star to aquire some distorted points and bars. Blinking quickly will remove the effect, while squinting briefly will bring it back again.

Beware of a misconception appearing in some books: if you look at the slit between closely-spaced vertical fingers, the tiny lines you see are NOT interference fringes. Instead, turn your hand 90deg, and the little lines go away. Or, squint your eyes while watching the lines, and they'll vanish ...which shouldn't happen with interference-bands. What's happening? The slot between fingers is acting as a pinhole camera, and the lines are images of your eyelashes. They mostly appear with a vertical slit, because eyelashes are mostly vertical. And here's a big clue: with real intereference, the fringes grow wider as the slit-width decreases. So, if you make the slit more narrow, eyelash-images don't change, but real interference bands will swell up wider, apparently magnified, as with the glass facets below.

A phenomenon of note: low pressure sodium-vapor lamps will clearly show all sorts of diffraction effects, including closely-spaced fringes of incredible detail. Merc vapor and incandescent streetlights give a different effect.

I once saw an interesting phenomenon which would have given Issac Newton nightmares. I was riding in the back of a relatives' car. They had one of those cut-glass faceted spheres dangling from the rear-view mirror. At a stop light, the sphere was in the sun and swinging back and forth. I moved my head to find one of the refracted beams, and was rewarded with an intensely bright pointsource which swept in frequency from deep red through yellow green violet and then reversed. I squinted my eye at it, and WOW! All sorts of minutely detailed diffraction frills appeared (caused mostly by eyelashes), and, as the crystal sphere swung slowly back and forth, the entire array of fringes expanded and contracted in synch with the color changes. They expanded as they went from blue-green-red, then contracted as they went red-green-blue. The visual spectrum covers about 2:1 frequency band, and sure enough, the diffraction patterns were swelling and shrinking over about a 2:1 ratio.

Why would Newton chafe at this? Newton apparently was a staunch believer in the particle nature of light (although I've heard opinions that Newton himself wasn't so rigid, it was the Newton-worshippers who came after him who saw the wave-theory of light as blasphemy). This swinging faceted globe in the sun was displaying numerous and interconnected phenomena which have a trivial explanation if light is waves, but would cause no end of grief for someone who is convinced that light is granular, without waves. Basic science discoveries made with almost zero equipment costs, no?

Here's another cool one-person demo, best done while alone. If you are in a car at night while it's raining, try plastering your face against the glass. Slide it along until your eye is filled with the huge blurry image of a single raindrop on the outside glass. Move until a distant streelight is captured by the distorting lens of the raindrop. The resulting image is full of incredibly fine diffraction fringes. These are a probably more-detailed image than you've ever seen before, since they do not depend on the quality of your eye's lens, and instead are caused only by lightwaves and shadows. The crazy caustics and interleaved diffraction fringes, all slowly morphing as the droplet slowly makes its way down the window, are something straight out of a dream. Don't let your loved ones catch you with your face all smashed and distorted on the glass, staring fixedly off into the distance for many minutes at a time, while humming old rock albums to yourself. Bad trip, man!
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