# If I can see someone's eyes, can they see mine?

Besides the obvious cases where I'm behind a "one-way" mirror or have goggles/glasses on: is there one where I can see someone's eyes, and they can't see mine?

• Comments are not for extended discussion; this conversation has been moved to chat, where you cannot see the eyes of the participants. – rob Feb 13 '17 at 22:56
• yep, if I look in the eyes of a blind person, he can't see my eyes. – Matthias Burger Feb 15 '17 at 13:36
• If you have someones eyes in a jar on your mantle.. – DasBeasto Feb 15 '17 at 16:35

Fermat's principle says that the direction of travel for any light ray can be reversed. Therefore there is always a line of sight between a pair of eyes in both ways.

If one person is in the dark, then only one person can see the eyes of the other. So there needs to be enough light reflected from both person's eyes for this to work.

• What if one of the participants are inside a BH event horizon? – peterh Feb 14 '17 at 22:45
• First we should feel sorry for the poor who has fallen into the black hole. I am not sure whether they could see anything before being crushed. In case they can, they could not do anything with that knowledge. So one does not have to be worried to be spied upon from inside the black hole. – Martin Ueding Feb 15 '17 at 7:35
• As far I know, the infalling observer, inside the EH, can see (a nonzero part of) the outer world. Although the outer world can see also him, but not inside the EH. Rather they see him as he nears the EH, being more and more redshifted. But he never falls in the BH in the time of the outer world. – peterh Feb 15 '17 at 10:59
• So one might be able to see the observer's eyes (from the past) but one cannot see any judgmental looks from the things they have seen while being inside the event horizon. – Martin Ueding Feb 15 '17 at 13:36
• Where does the Fermat's principle actually say the path can be reversed? Because there is one magneto-optical effect, the Faraday effect, that allows rotating polarisation of light in opposite directions depending on direction of travel and then the paths can easily diverge. Combining it with polarisation filters to create one-way path is fairly straightforward (combining it with birefringent wedges and collimators to create one-way path for light of any polarisation is slightly less obvious, but possible). – Jan Hudec Feb 15 '17 at 17:12

The answer of Martin Ueding is correct if there is no intermediate image in the light path. For example, if you use a camera obscura, in general there will in be no way for the observed person to create an image of your eye.

So the answer is NO for direct light paths and YES if you allow intermediate images.

• I did not think of that! If you have a non-reflecting screen somewhere, it is a one-way thing. Another thing I just though of would be using a telescope or a doorspy. In prinziple, one could see the other person's eye, yet it is much harder for the other person. Also if I look at a photo of somebody, I can see their eyes although they cannot see mine. That probably is included in your answer as there is an “intermediate image”. – Martin Ueding Feb 12 '17 at 17:45
• @MartinUeding I did not want to talk about more advanced optics, but there are optical isolators which are based on the magneto-optic effect. So puristically speaking the answer is actually yes in all cases :-) – user_na Feb 12 '17 at 17:57
• I've used an optical isolator in one lab course myself, interesting stuff! I was just thinking about two people looking at each other in the vacuum ... being a theoretician has its disadvantages :-). – Martin Ueding Feb 12 '17 at 18:04
• @rob if any further pedantic arguments occur I will make use of my optical isolator or my optical meta material to let this post disappear. – user_na Feb 12 '17 at 18:19
• Funny thing about pedantic, I have been accused of being just that over on Academia. But if you look at the definition: Of or like a pedant, and then look at the definition of pedant: a person who is excessively concerned with minor details and rules or with displaying academic learning....I came to the conclusion that being called Pedantic can actually be a compliment. – NZKshatriya Feb 13 '17 at 5:03

Profiting from the fact that $c$, the speed of light, is finite, one could build a "monodirectional telescope" by using two shutters separated by some distance $L$.

The shutters stay closed for a time of $2L/3c$ and open for a time of $L/3c$, the opening time of the second being delayed by $L/c$. The photons entering the telescope from the first shutter will find the second open, while from the opposite direction it will be closed.

The fact that $c$ is fairly large does not make things particularly easy or practical, but it would work. Note that a similar apparatus can be (and has been) used to measure the speed of light.

• This seems cool. How small can this be built as a practical device? I.e., how good are we at controlling high-speed shutters? Also, with more than two shutters, the efficiency could be raised from 1/3 to almost 100% – Hagen von Eitzen Feb 13 '17 at 17:33
• @HagenvonEitzen Thanks for noticing that one can increase the efficiency with more shutters! With a fast spinning cogwheel one may reach microsecond-level control depending on the required aperture (aka distance between teeth) but $c=300~m/\mu s$... There might be some electro-optical phenomena with faster reaction times, but I leave the stage to more expert people. – DarioP Feb 13 '17 at 18:27
• Could you achieve this by attaching two fins in series (that let light through), with appropriately placed cavities, along a rapidly revolving axis? Would this be more reliable than two separate shutters alternately opening? – Tobi Feb 13 '17 at 20:16
• This is an incredible idea but I believe it is still susceptible to the ladder paradox in the case that the subject is moving at relativistic speeds with respect to the observer/telescope. Of course, this could be determined through non-optical means before the observation is made and the timing of the shutters could be adjusted accordingly to preserve the integrity of the design. – Jaquez Feb 14 '17 at 20:14
• @DarioP, it can be built about 1 m long, perhaps half, using Kerr cells. I've seen these used in high power laser where it opened for 1 ns to let a 30 cm laser pulse through and the facility since upgraded to at least 0.5 ns. The transition time is around 0.1 ns. – Jan Hudec Feb 15 '17 at 16:57

Martin is correct, but neglects the case of distance. If you use a telescope or binoculars, you may have line of sight with the other person, but the other person may be too far away to "see the whites of your eyes". This is a function of how well you can resolve distant objects.

I'm not going to use the "observation by electronic camera" answer since in that case you are not actually seeing their eyes but rather a representation of their eyes on your monitor.

• Or the other guy may be short-sighted – Ángel Feb 13 '17 at 0:49
• Or the other "person" could be an eagle or superman, both of whom have telescopic vision :-) – Nav Feb 13 '17 at 5:40
• This isn't correct. The diffraction limit on the angular resolution of a telescope is inversely proportional to its aperture, so e.g. doubling the aperture doubles your ability to resolve the other person, but also doubles their ability to resolve your eye (/telescope aperture). The relevant limits are symmetric, so if you can resolve their aperture, they can resolve yours. Mind you, things like telescope arrays make this more complicated. – Gordon Davisson Feb 14 '17 at 6:24

High power lasers use a contraption, called Faraday cell, or optical isolator, that allows propagation of light in one direction, but not the other.

The device consists of a Faraday rotator and two polarisation filters. The rotator uses Faraday effect, which rotates polarisation of light under magnetic field in suitable medium by angle depending on strength of the magnetic field in direction of the propagation of light, so returning light is rotated in opposite direction. Around the rotator are two polarisation filters rotated by 45° and the rotator is adjusted to turn the light by 45°. In one direction that allows the light to pass, but in the opposite direction the light arrives at the second filter 90° out of plane and is absorbed.

The wikipedia article also describes polarisation-independent variant that uses birefringent wedges instead of filters. In one direction, the light is properly recombined, in the other it is diverged and blocked by a collimator.

In the lasers, it is used to prevent reflections going back to earlier stages and causing additional pulses or even damaging those stages—high power laser is composed of oscillator that creates the initial pulse (say, 0.5 ns long) and several progressively larger amplifiers and the early stages are not designed for the powers at the end of the optical path.

The Faraday isolators are also used in optical communications.

The power lasers also use other elements, Pockels cells and Kerr cells. Both are blocks of suitable material that rotates polarisation only when electric field is applied. The cells are again guarded by polarisation filters so the light can pass only if appropriate electric field is applied. The rotation is reciprocal here, so light can pass both ways, but it is used to quickly open and close the optical path.

Usually a Pockels cell is used to split the laser cavity until the medium is charged and then connect it so the pulse can start building up and then a Kerr cell, which is faster, but needs (much) higher voltage, is used to let the beam out for the desired 0.5–1 ns.

This allows to build the device suggested in DarioP's answer measuring about 1 m.

• a camera and a TV screen also make for a good optical isolator :-) – Florian Castellane Feb 16 '17 at 11:55
• @FlorianCastellane, that basically falls under the intermediate image discussed in user_na's answer. However a screen (whether TV or just projection) does not pass the original light, so lot of information is lost. Faraday isolator passes the original light. – Jan Hudec Feb 16 '17 at 13:15

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