Line of vision - If I cannot see someone, is it possible that they can see me?

Imagine I'm on a rooftop of a building. If I am lying down behind a wall and cannot see someone standing on another building- no matter which direction I look, is it possible that they can see me?

• It's possible they can see your feet, at least. – The Photon Jul 14 at 21:43
• But definitely not the face, right? I tried to reason this out and to me it makes sense that they wouldn't be able to see my face given that the direction of travel for any light ray can be reversed. Am I right? – worriedsickly Jul 14 at 21:45
• Also - please note both people are in full light – worriedsickly Jul 14 at 21:50
• It's true that "light rays can be reversed", but that doesn't necessarily mean that if light comes off an object and then into your eye that light must also be coming off from your eye to that object. If I shine a laser pointer on you does that mean you are shining a laser pointer on me? – Aaron Stevens Jul 14 at 22:41
• Yes, you are safe in a shower with windows open, as long as you can't see anyone outside ;) – safesphere Jul 15 at 0:38

The basic answer to your question is "no, absent weird effects like exotic polarized fields", but it's worth noting that when the sun is fairly low and one person is "upsun" and the other is "downsun", and they are looking at each other, the "downsun" person will appear sharp and crisp with all colors and details standing out brightly, while the "upsun" person will be a dark silhouette surrounded by blinding glare. People forget this all the time, and think each can see the other equally well. That's why the eastbound drivers don't bother to turn on their headlights at sunset, even though the oncoming westbound drivers are squinting like crazy into the glare and can barely discern anything at all.

Let's assume static conditions, otherwise the question doesn't really make any sense. Under static conditions, if light can travel in one direction along a path then it can travel in the opposite direction along that same path, except for a polarization issue which I'll discuss in a moment.

For conditions where polarization is not an issue, the logic goes like this:

1. If A can see B then B must be able to see A (because light can travel in either direction along a given path). I'm assuming equal illumination at each place.
2. Therefore, if B can't see A then A can't see B.

However, polarization can change this outcome, because by using polarization you can construct a one-way valve for light. It consists of a rod with a large Faraday affect in a strong magnetic field, so that light polarization rotates by 45 degrees when it travels down the rod. Polarizing filters are added at either end. In this configuration light traveling in one direction gets rotated by 45 degrees and thus is aligned with the polarizer at the exit and comes out, but light traveling in the other direction rotates 45 degrees and thus is brought to being orthogonal to the input polarizer and gets absorbed. Such a device is called an "optical isolator" and is widely used in high-precision atomic physics labs.

More generally, this kind of effect can be used to steer beams in different directions depending on the combination of their polarization and direction of travel.

There are many cool things one can do with light. For your case, think about one-way mirrors famous for their use in police interrogations. The cops standing in the control room can see the interrogation through the one-way glass whereas the criminal being interrogated cannot see them.

https://en.wikipedia.org/wiki/One-way_mirror

Yes,it's possible. I have often seen people who couldn't see me. The classic example is the ostrich which buries its head in the sand when it sees danger approaching, or so the legend goes. It's actually a misrepresentation, but it's not entirely a myth. The ostrich doesn't actually bury is head in the sand, but when it sees danger while sitting on its nest, it lays its head and neck straight out along the sand to lessen its visibility. From a distance it might easily seem to an observer that its head is buried in the sand. It probably thinks that if it cant see the predator, the predator can't see it. Even humans sometimes think like that.

• Interesting ostrich observations but does not grasp or answer the question asked, which was about optics not psychology. – Andrew Steane Jul 15 at 7:47