# Seeing through windows – light and dark

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.

1. #1 6EQUJ5
January 14, 2010

When I was a little kid I was amazed to realize that when I was up inside a tree (in the summer) I could see somebody looking for me but they couldn’t see me. It took a while for me to work it out that I was in deep shade, hidden behind the glare of the leaves, making me invisible, while he was in broad daylight.

It was only a little later that I waved through the window to somebody outside through, but they couldn’t see me until I moved up close to the window.

2. #2 Anonymous Coward
January 14, 2010

Nice connection of how one-way mirrors work to everyday experience.

In my evil-scientist-with-enormous-financial-resources dreams, I make all my one-way windows using faraday rotation and polarizers. True one-way viewing! Of course, in my dreams I’ve also invented a broadband faraday rotator.

3. #3 csrster
January 15, 2010

Net curtains are so sneaky. When you can’t see out of them, that means other people _can_ see in.

4. #4 PrivateTutor
January 15, 2010

For one way glass the surface of the it is so shining with extra polishing materials that effect on reflection methods. If you are outside and inside room is having same brightness then also you ca not see any objects inside the room. because the shining surface does not allow the sunlight ray to pass through the glass. It is completely reflected.

Views from Physics Tutor

5. #5 Art
January 15, 2010

I very informative piece.

Okay, can you explain how the two-position rear-view mirror works? You know the device, some idiot drives behind you with his high-beams on. The light from his headlights, reflected from your rear-view mirror, blinds you. You reach up and flip the little lever that tilts the mirror in a specific way and the light is no longer in your eyes but you can see behind you. A bit dimmer but you can still maneuver.

6. #6 rob
January 15, 2010

off the top of my head, without resorting to looking at my rearview mirror, i’d say that the two position rearview mirror in normal position reflects light mostly off the shiny surface off the back of the mirror. when you flip it, you change the angle of the shiny mirror so you don’t see the refllction. instead you are seeing the reflection off the glass front of the rearview mirror.

since shiny mirrors reflect near 100% and glass only about 7 or 8%, the bright lights look dimmer in the flipped position.

7. #7 Art
January 16, 2010

That sounds good Rob, I appreciate your trying, except for one small catch. Unless the mirror was made in a very special way, perhaps if it was wedge shaped in cross section, wouldn’t the front and back surface of the glass be parallel? And if parallel how would tilting the mirror, which is what the lever does, shift the reflective surface used from one to the other? Or have I failed to understand some basic concept?

I feel a bit foolish not knowing what must be some very basic scientific principle that allows this simple device to work but it remains quite mystifying.

8. #8 magetoo
January 16, 2010

To expand on what rob was saying:

The shiny mirror surface is still there, and would still be reflecting light better than the “secondary mirror”, but of course now it’s pointed away from the rear window (pointed up?) and there is less light reaching your eyes by that path than light coming in through the rear window and reflecting directly off the secondary mirror. That’s why you still see the car behind rather than the interior of your own.

(The details of how you switch between the two is engineering, and therefore not a “catch”, but some other blog’s problem.) ðŸ™‚

9. #9 anonymous Coward
January 17, 2010

Re: rob and Art at 6 & 7

You’re both right of course – the glass is wedged. You can verify this yourself by moving your head up and down to see both reflections.

Unfortunately I can’t do this myself in my current car – I’ve got one of them newfangled auto-adjusting mirrors. Soon the toggleable glass wedge rearview mirrors will be a thing of the past.