We had a running joke in science ed that kids get so overexposed to discrepant events involving density and air pressure that they tend to try to explain anything and everything they don't understand with respect to science in terms of those two concepts. Why do we have seasons? Ummm... air pressure? Why did Dr. Smith use that particular research design? Ummm... density?
I think we need another catch-all explanation. I suggest index of refraction.
To simplify greatly, index of refraction describes the amount of bending a light ray will undergo as it passes from one medium to another (it's also related to the velocity of light in both media, but I do want to keep this simple). If the two media have significantly different indices, light passing from one to the other at an angle (not perpendicularly, in which case there is no bending) will be bent more than if indices of the two are similar. The first four data points are from Hyperphysics, the final one from Wikipedia... glass has a wide range of compositions and thus indices of refraction.
Water at 20 C: 1.33
Typical soda-lime glass: close to 1.5
Since glycerine and glass have similar IoR, light passing from one to the other isn't bent; as long as both are transparent and similarly colored, each will be effectively "invisible" against the other.
So, why does it rain? Umm... index of refraction?
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