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The Physics of .....Pink Floyd?

Apparently its 50 years this March since Pink Floyd released the album, 'Dark Side of the Moon'. I know from experience that teenagers today would struggle to name a single track on the album, but that many will recognise the iconic album cover. Perhaps more impressively, they'll be able to name the album and band if shown the artwork - even though neither appears anywhere on the cover.

In terms of physics, I'll skip right past the whole 'there is no dark side to the moon' thing. I think that's well understood and it's even referenced on the album itself where a background voice says 'there is no dark side to the moon, really.' Though unfortunately it follows up by saying 'as a matter of fact, it's all dark....'

But there are two interesting aspects to the design that are directly relevant to Leaving Cert Physics. One is how - if the original gatefold design is fully opened up - you see the image shown above. Where the light is not only split up into its constituent colours by passing through a prism, but also recombined into white light by a second prism. Apparently this was done to allow interesting displays in record shops, but it neatly invokes one of Isaac Newton's earliest contributions to physics.

The ability of blocks of glass to split light into a band of colours was well know before Newton. In fact, we know that in Newton's time, pieces of cut glass would have been available in markets, sold as trinkets precisely because of this effect. But at the time, few understood what was happening, and most thought the glass was somehow adding colour to the light, or at least changing the white light it in some way as it passed through. Newton showed this was not the case - that the various colours were all there all of the time - by setting up a second prism which took the multicoloured band of light and recombined it into white light. Just as the Pink Floyd image demonstrates.

But there is another aspect of the album's artwork that is worthy of attention in physics classrooms. And that is to ask what is the refractive index of the prism illustrated in this image, and does it correspond to any available material?

To figure this out, I printed out copies of the artwork and enlisted the help of some Leaving Cert students. We added in normals where the light strikes the prism and where it emerges, and carefully measured the various angles of incidence and refraction, allowing us to calculate values for the refractive index. Or should I say refractive indices - because what we discovered was somewhat disturbing.

Having more than one refractive index isn't a problem in itself. Just looking at the two extremes of the spectrum, the refractive index for the violet light has to be greater than that of the red light. That is why, after all, the light separates into its different colours: violet light slows down far more than red light when it enters a dense material, and that is why it bends through a greater angle. I had checked out typical values in advance and knew that for red light passing through glass the value of the refractive index is usually about 1.51 and that for violet light it is about 1.53. But the Dark Side of the Moon 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 far too high to be glass, but - a little research showed - it matches the refractive index of zincite - a transparent mineral that mainly contains zinc oxide. But the same research showed us that Zincite is usually either yellow or red, so it hardly matches the image in the photo, 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 briefly if variations in the density of the prism could account for the inconsistencies, 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) were varying.

It just doesn't make any sense. Its almost like the artists involved had no respect for 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...

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I'm sharing a module I've put together that I use in TY as a (very basic) introduction to Applied Maths - concentrating on where that overlaps with Leaving Cert Physics. It's still a work-in-progress,

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