Here is a picture of something you have seen before.
These are two pictures of the same location in my house. The one on the left is taken when the Sun was up outside and the other one when it was dark outside. For both pictures, I had the same lights turned on inside. So, why does it do this? Why can you see stuff outside when it is bright outside, but you don’t see a reflection of the stuff outside? Why when it is dark outside, does the opposite happen? You know what I am going to do next, right? Diagram. Here is a diagram for when it is dark outside.
- The person can see the blue box because the light from the lamp reflects off of it (I drew that light as blue arrows).
- When light hits the window, some of it goes through the window and some reflects off the window.
- Some of the light reflected from the window goes to the person so that the person can see a reflection of the blue box and the light.
- Some of the light goes through the window. So, if you were outside in the dark, you could see the light and the blue box.
Now, what if it is not dark outside.
Now there is a lot of bright light outside (from the Sun). This light reflects off the giant red monolith that just happens to be there. Again, some of that reflected light goes through the window so that the person can see it. Some of that light reflects off the window also. The person inside can see the monolith, but this light is too bright for the person to see the reflections from the inside of the room (but they are still there).
If the inside light is not too bright, a person on the outside could not see inside because the light reflected from the outside is too bright. The light from the inside is still getting out, it is just too difficult to detect.
This is essentially how a one-way glass works.