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