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I have toyed with the idea of explaining observations of everyday physics for quite some time, so let me just start. This post features two pictures of reflections of light in a modern window. I hope to point out, what can be learned from these simple observations.
The red LED of a simple power supply placed on a window sill is seen towards the bottom in this picture:
/file/20240315_Glass_Panes.jpg
There is a reflection of the LED in the upper left part of the image. Well, actually, there are 3 groups of 2 reflections each! The three groups correspond neatly to the fact that the window was manufactured with three sheets of glass. The pairs in each group correspond to the fact, there is a reflection on both the front and the back of each sheet. Nice!
So, in order to find out, if your window at home has one or more sheets of glass, all you need is a small light source, like a LED.
In the stairway of an office building I noticed the reflections of LED stripes running vertically along the wall in a fairly large window oriented perpendicular to said wall.
/file/20240315_Glass_Panes_warped.jpg
There is a light stripe just adjacent to the window, and its reflection can be seen just inside the window frame. However, there is a second light stripe at the other end of the wall. Its reflection is seen in the window as well: the reflection appears thinner, threefold, and warped! As above the window is manufactured with three sheets of glass, thus we expect the reflection to be threefold, indeed. But honestly, I did not expect the pronounced bends. This effect can be attributed to the sheets actually being warped. This is not entirely surprising, since the glass sheets are glued to a spacer along their edge in a gas tight fashion. And the sheets act as membranes reacting to the pressure difference between the outside (local air pressure) and the inside of the window (local air pressure at time of sealing). So over time one could expect tiny variations due to changing air pressure, which actually makes for a barometer of sorts. It is not easy to get a reading, though. Additionally I expect that the glass sheets warp under their own weight, and that effect could well be of similar size, but I don't know any better.
Now that I think of it, the amount of warping is probably also changing with temperature. So it is a combined thermo-baro-meter, not easy to get a reading or a good calibration, for sure.
