Eye Contact

June 16, 2005 8:07 AM

PhysicsFilter: Suppose I'm looking straight ahead at somebody's head, but can't see their eyes because they are obstructed by a wall or railing. Under what physical circumstances can that person see my eyes? Assume that there are no mirrors, cameras, or other visual trickery. Also assume that both people are sitting on chairs at floor level, though they may be of different heights.
posted by Saucy Intruder to Science & Nature (17 answers total)

What is the wall or railing made of? Something transparent?

How about something offset that would refract the light around the wall?

Pinhole camera effect in the wall?
posted by 5MeoCMP at 8:28 AM on June 16, 2005

It's easy to understand that our ears are passive--they just sit there and receive sounds, either directly, or bouncing off stuff. But because light travels much faster than sound, it's more difficult to realize that our eyes are also passive. They don't go out and get colors and patterns and objects; they just receive echoes of light that starts in the sun or the room lights, and bounces off things (with some wavelengths subtracted in the process). It travels in straight lines. If a person sees your eyes, you must see theirs, unless there is something like glass or fabric between with differential lighting. A translucent blindfold would do it. Gravitational lensing or other refraction wouldn't do it, because light could travel both directions on that path.
posted by weapons-grade pandemonium at 8:29 AM on June 16, 2005

Unless you're way off the the side where they can't see you in their peripheral vision, or unless they're just not looking in your direction, if you can see their eyes, they can see your eyes.
posted by bshort1974 at 8:32 AM on June 16, 2005

Is this a puzzle of some sort? They could be blind.
posted by weapons-grade pandemonium at 8:34 AM on June 16, 2005

...it's more difficult to realize that our eyes are also passive. ... they just receive echoes of light that starts in the sun or the room lights, and bounces off things ... It travels in straight lines.

Except, light dosn't always travel in straight lines, can be diffused around corners, etc, but I don't think this comes into play at all here.

If the railing had small holes in it, and was close to the other person, they might be able to see through it, while you cannot.

But if the obstruction is totaly opaque then I do not think that they can see your eyes either.
posted by delmoi at 8:34 AM on June 16, 2005

In fact, even if there are mirrors around the rule holds true. If you can see their eyes, they can see yours unless there is some one-way sheild like a pair of sunglasses
posted by delmoi at 8:37 AM on June 16, 2005

if you can see their eyes, they can see your eyes

Only if there is a straight line connecting pupil to pupil, and pupils can be different sizes. You are both seeing through your pupils, but the following assumes that "see my eyes" means "see any portion of my eyes."

Let's call the people A and B. If A can not see B's eyes, B might still see A's eyes in two possible ways:

(a) If A's eyes are bigger (in the vertical dimension) than B's eyes, B might be able to see the top portion, above the pupil, of A's eyes (this assumes the obstruction is situated so that A's view of B's eyes is cut off just above the top of the eyeball); or

(b) If the lighting behind B is quite bright, while the lighting behind A is quite dim, then B's pupils will be open much wider than A's. The wider aperture would allow B to see the top portion of A's eyes, though not the pupils (because if there is a straight line pupil to pupil then A would be seeing B's eyes).

Other than that, I see no way, assuming that "no visual trickery" means no dark glasses or anything else creating a one-way viewing path.
posted by beagle at 8:38 AM on June 16, 2005

Response by poster: Not a puzzle - just in a curious mood. The blindness thing is clever, but I guess we have to assume that both people have the same level of visual acuity.
posted by Saucy Intruder at 8:41 AM on June 16, 2005

If the railing had small holes in it, and was close to the other person, they might be able to see through it, while you cannot.

This sounds right to me. And you would not be able to see their eyes.

Also, say I have a long cardboard tube (like from wrapping paper or something), and hold it to a hole in the railing or wall. I'll be able to see your eyes through the hole/tube combo, but you will not see mine. The same principle applies without the tube if I'm able to get close enough to a hole to see through, but you are not.
posted by jdroth at 8:45 AM on June 16, 2005

OK Saucy Intruder, you're a law student.
How about, "I don't recall seeing their eyes."?
posted by weapons-grade pandemonium at 8:52 AM on June 16, 2005

To me, pinholes, tubes, mirrors, glasses, blindfolds, etc all fall under "visual trickery" which is excluded.
posted by beagle at 9:07 AM on June 16, 2005

...and seeing the side of somebody's eyeball falls under "verbal trickery", which also should be excluded.
posted by weapons-grade pandemonium at 9:25 AM on June 16, 2005

If you took things down to a quantum level, and defined "seeing" to include the brain processing information from the retina, there would be a miniscule difference in the processing time for each individual, if only based on a difference in length of nerve paths, or number and condition of neurotransmitters. The person who processes first "sees" the other first (for a googolth of a second). There is also a persistence of vision (afterwards) which may vary for each individual.
posted by weapons-grade pandemonium at 9:38 AM on June 16, 2005

If you can't see my mirrors, I can't see you!
posted by knave at 11:47 AM on June 16, 2005

to a first approximation, if you can see their eyes, they can see yours (you both might do so via a mirror). details others have mentioned include exactly which part of the eye, and the details of processing in the brain.

more exactly, light scattered off their eye that gets to your eye can take the same path as light scattered off your eye, detected by their eye. since light travels in straight lines, the light going to each person's eyes goes along the same path, but in opposite directions.

that explains why things are not symmetric if one person is in a dark room - there is no ambient light to scatter off their eye, so you will not see their eyes, even though they can see yours.

sunglasses work similarly, in that the person wearing the sunglasses has all incoming light dimmed, so can still see the other person's eyes, unless everything is too dark. but going in the other direction, only their eyes are dimmed, and the person "outside" the sunglasses receives more light reflected from the glasses than they do from the eyes, making it difficult to see the eyes behind the glasses. it is a combination of light reflected from the glasses themselves masking the eyes behind them, and the attenuation of the light scattered from the eyes.

but before you decide that this means it's ok to eye up people when you can't see their eyes, be careful. we seem to be very good at reading very indirect clues about where people are looking - the attitude of the head, the way the body is held, etc. so even if they can't see your eyes, they may "sense" that you are checking them out (or a friend may tell them...)
posted by andrew cooke at 11:59 AM on June 16, 2005

There is one exception to the light-can-go-both-ways rule: if they are sitting inside the Schwarzschild radius of a black hole, and you are sitting outside, then they can see your eyes and you cannot see theirs. But if you put up a wall, they still can't see through it; so, I like delmoi's little-holes proposal.
posted by nicwolff at 12:33 PM on June 16, 2005

Basically, are we just looking for proof of the "If I can't see you, you can't see me" principle?
posted by Faint of Butt at 12:36 PM on June 16, 2005