|MadSci Network: Physics|
Hi, Eric! I don't know the specifics of your window (how many panes, what kind of coatings, etc.), or, the lighting of the room you are in. Also, I'm kind of writing in "hurry-up" mode, as I've already taken far too long answering your question. Nevertheless, I think I know what you are talking about, so here goes… Reflection of light from window glass is a tricky thing. It depends upon your viewing angle, the angle of light coming from the (inside) source, the intensity and spectrum of the light both inside and outside, and a whole bunch of other factors like coatings, layers of glass, etc. Imagine you are standing in a room lit by a single lamp-bulb. Let's say that bulb is equidistant between you and the window, maybe off to your right side by a few feet, so we have a nice triangle between you, the lamp, and the window. (Try drawing this as a view from above.) Some light rays leave the bulb and bounce off the window back into the room. An equal number might leave the bulb, bounce off you, and go out the window. Some of these rays (plus the ones reflected off the window, back to you, then back to the window) never make it out the window, but get reflected back to your eye, which "gobbles them up" and creates the image you see. Let's further assume that about 95% of the light hitting the window goes straight through to the outside. That would mean only about 5% of the light hitting the window from the inside can actually be reflected back into your eye. (Those figures aren't exact. Like I said, there are lots of complicated factors involved here. 5% is a pretty good "ballpark" figure to start with, though.) Now, let's say you take a few steps toward the window, so that you are in line with the lamp, but still right in front of the window. Fewer light beams leave the bulb, bounce off you, and get reflected from the window. Why? Remember the triangle I mentioned before? One corner represented the window, one the bulb, and one you. If you draw this triangle, then move the "you" corner towards the window corner so that it stops in line with the "lamp", half our triangle has disappeared. This is important, because we often think of light "rays" as straight lines. If you draw a whole bunch of lines from the "lamp" corner to where "you" started, and keep drawing lines until you hit the new "you" (moved halfway towards the window), you will color in half your original triangle. Think of these as light rays which "can't" hit you any more, but have to bounce off something else in the room. Less light bounces directly off you and goes to the window, so, less light can be reflected back to your eye. Let's "cheat" and say only half the light hitting you before you walked toward the window is hitting you now that you are halfway there. If you were in a room with one bare bulb and nothing but black curtains for walls, this might be about the case. (I'm simplifying the math, here. The important thing is: the light which did not hit you would be absorbed by the curtains, never to be seen again.) You might find you could only see half your face in the reflection, because the other side would be away from the bulb, therefore, in shadow. You started off seeing 5% of the TOTAL light inside the room reflected off the glass. You've just cut that down to 2.5% (half) by moving towards the window. Now, let's say the light coming from outside is only 2% the intensity of the TOTAL light inside the room. The amount of light reflecting from inside the room is getting very close to that coming from outside the room…the intensity of light reflected off you is nearing that which comes from outside the window. By getting closer to the window you are cutting down the angle of light which can reflect off you and to the window. Take another step…then another. Fewer and fewer light rays can bounce directly off the bulb, hit you, then be reflected off the window into your eye. Neither the outside, nor, the inside light changes one bit. The light rays and reflections are still there. You've just changed your viewing angle, and, therefore, the relative amount of each "kind" of light hitting your eye. There are a couple of other ways you can demonstrate this for yourself. First, try taking a flashlight into a totally dark bathroom. Shine it directly on your face and look at your reflection in the bathroom mirror. Now, shine the light straight up from under your chin. Less light bounces off your face and into the mirror, so you see some really cool, spooky, "dark shadows" on your face. Another thing you could try is looking in a car's rearview mirror at night. Most of these mirrors have a switch setting for day, and for night. All this does is tilt the mirror by about 5 degrees…you can feel it change with your hands. That's enough to make sure you only see the small percentage of light which bounces off the surface of the glass, instead of the other 95% of the light which would normally bounce off the mirrored back surface of the glass. (It would probably be good to have someone else do the driving for this experiment. You also don't want to do this in heavy traffic, as your driver will probably need to use their rearview mirror to keep from hitting people. Make sure to ask your driver politely in advance, because safety on the road is ALWAYS much more important than this experiment! Maybe if you were at a stop light? ;) I've only been able to explain a little bit about this, Eric. There's so much more to know! Just thinking about this question could lead you to investigate: reflection, refraction, basic optics & physics, scattering, the nature of light and the way humans perceive it, the human eye, different kinds of math like trigonometry and calculus, quantum theory…and a whole bunch of other stuff! (That's why this is such a good question. It makes you think. If you think you don't know the answer to a question, then you might have to learn some stuff to get your answer…which leads to understanding of a lot of things you didn't even think about in the first place! Ask one good question and, "boom"! You're a genius!) If you would like more details / references, or, if I've been unclear (or, wrong), please feel free to e-mail me at firstname.lastname@example.org. I not only like your question because it teaches us all something, but because I always wondered about the same thing myself when I looked out the window on an overcast afternoon. I'd be happy to help with some more research on this great mystery of the universe! Your friend & MadSci, -Matt email@example.com firstname.lastname@example.org
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