Date: Sun, 30 Oct 2011 15:40:40

Author: --- William Beaty

Subject: Re: Optical Trick - can someone explain?


On Sat, 29 Oct 2011, Paul Nord wrote:
> The other evening my kids discovered a curious thing. If you look
> through a drop of water on your glasses at a distant light source you
> will see an array of curious objects floating around. I tried it myself
> and discovered that you always get the same set of floaters - different
> drop, different spot on the glasses, same floaters. It would seem that
> the floaters must be inside the eye, or possibly on the surface of the
> eye.
> Can anyone have a good quick explanation of the optics involved here?

They're shadows of microscopic debris. But since pointsource light is
coherent light, we end up with a sort of "phase-contrast microscope"
effect which makes transparent objects visible. In my experience, the
textbook explanations of Coherent light are just terrible. They usually
don't mention that Coherent light is just point-source light, that
starlight is far more coherent than laser light, and that "spatial filter"
just means "pinhole."

The water drops form real-images of the point sources (pull your glasses
away from your face to see these tiny sparks of light.) The images behave
like any tiny object held too close to your eye: each forms a large

Similar effect: on a summer beach with overhead sun, cover your face with
a dark towel. Each blur-disk of sunlight from pinholes in the towel will
contain the pattern of floaters. Or, get a very small pinhole aperture
plate, put your face near a large frosted light bulb, then observe the
bulb with the pinhole almost touching your eye.

For awhile you could get "floater viewers" from Edmund Sci. I never had
one of these to disassemble. I bet they were just an LED with a pinhole
plate, or perhaps an optical fiber.

(((((((((((((((((( ( ( ( ( (O) ) ) ) ) )))))))))))))))))))
William J. Beaty
beaty, chem washington edu Research Engineer
billb, amasci com UW Chem Dept, Bagley Hall RM74
206-543-6195 Box 351700, Seattle, WA 98195-1700