Basically the title. Light can go into my room through some lenses so that I can see it, but light can't escape my room so that people outside? The article about peepholes on Wikipedia doesn't elaborate at all about how this works.

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    $\begingroup$ Have you ever looked the wrong way through binoculars? Or any lens, for that matter? Why would you think looking through a lens has to be the same from both sides? It's not the amount of light going inside/outside, it's the way the lenses deflect it. $\endgroup$ – ACuriousMind Dec 31 '15 at 3:07
  • $\begingroup$ I have never. On optics lens diagrams, we draw light going from one way and exiting the lens the other way, and if it went backwards everything would still work? $\endgroup$ – Faraz Masroor Dec 31 '15 at 3:08
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    $\begingroup$ Why the downvote? $\endgroup$ – Faraz Masroor Dec 31 '15 at 3:19
  • $\begingroup$ Passive optics always pass light both ways. There is a fascinating little bit of action in Leon in which our hero avoids using "the" peephole and peers into the hall through a secondary one instead because you can see the change in light when a person approaches a standard peephole. $\endgroup$ – dmckee --- ex-moderator kitten Dec 31 '15 at 3:38
  • $\begingroup$ Downvote because you've failed to justify your claim. Light goes both ways just fine. $\endgroup$ – Carl Witthoft Dec 31 '15 at 13:43

Peepholes are usually wide angle lenses. This allows the home owner to see all corners of their front porch, or for hotel room guests to see practically the entire hallway. Wide angle lenses capture more light than a flat lens because light coming from the sides can still impact it and be refracted. The peephole lens then condenses all of the light it collects down into a tiny stream that is then fed into the eye of whoever is looking through it.

When you look through the peephole from the outside, however, this effect is reversed. The eye tries to collect a wide angle of light coming from a small stream from inside the room. The effect is that whatever you are seeing looks tiny, so you cannot clearly see the face of whoever is inside the house.

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