It may not be the most pressing issue in science right now, nor the most important thing about Pink Floyd's album 'Dark Side of the Moon,' but I found myself wondering recently: what is the refractive index of the prism illustrated in this image, and does it correspond to any available material?
So I printed out a copy and reached for my protractor. I added in normals where the light strikes the prism and where it emerges and I carefully measured the various angles of incidence and refraction, allowing me to calculate values for the refractive index. Or should I say refractive indices - because what I discovered was not pretty.....
Obviously I expected more than one answer. Just looking at the two extremes, the refractive index for the violet light has to be greater than that of the red light. I had checked out typical values in advance and knew that for glass and red light the value is usually about 1.51 and that for violet light it is about 1.53. But Pink Floyd's image doesn't produce values anything close to either.
On the way into the prism, the angles of incidence and refraction for the violet light yield an answer of 2.42. That value is too high to be glass, but it matches the refractive index of zincite - a transparent mineral that mainly contains zinc oxide. But Zincite is usually yellow or red, so it hardly matches the image in the photo above, and I doubt it could be easily cut into the prism illustrated in the diagram.
That doesn't really matter, though, because the material used can't be Zincite. Or anything else. If it was we could get a similar, though slightly smaller, value for the red light. We don't. For the red light, we get a value of 1.15. Which doesn't correspond to any common material that I can track down.
It gets worse. When the light emerges to the right of the image, the angles measured there give us two more, entirely inconsistent values: 1.08 for the violet light, and 1.85 for the red.
I wondered if variations in the density of the prism could account for the inconsistency, but that doesn't work: we can see the path followed through the glass, and that should be curved if the density (and refractive indices) are varying.
It just doesn't make any sense.
Its almost like the artists involved had no respect Snell and his laws of refraction!
Incidentally, I came across this story here, which shows us what the image would have looked like, had it been realistic. Still looks quite pretty to me...