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?
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:
- 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.
- 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.
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.