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Everyday science

Great gaffe in the sky: the erroneous physics behind The Dark Side of the Moon

02 Jun 2023

Tom Tierney turns the famous album cover into a classroom physics exercise, and uncovers some inconsistencies

Prism scattering light
Artistic licence If the physics had been more accurate, the Pink Floyd cover might have been a little less iconic. (Courtesy: Mason Maxwell @masonmax98)

This year marks 50 years since British rock band Pink Floyd released their seminal album The Dark Side of the Moon. From my experience as a physics teacher, I can tell you that most teenagers today would struggle to name a single track on the album. But a majority of them still do recognize the iconic album cover, which depicts light refracting through a triangular prism. Indeed, I am convinced that students will be able to name both the album and the band if shown the artwork (even though neither appears on the front cover) making it a useful tool in the physics classroom even today.

In terms of actual physics behind the art, I’ll skip right past the fact that the Moon does not have a true “dark side” – simply a “far side” that we cannot see from the Earth, as the Moon is tidally locked.  Amusingly enough, this is even referenced on the album itself where a background voice says “there is no dark side to the Moon, really” before adding “as a matter of fact, it’s all dark…” This is, perhaps, a nod to the fact that the Moon does not produce its own light?

Setting these astronomical facts aside, there are two interesting aspects to the design that are directly relevant to physics students. One is how – if the original gatefold design is fully opened up – you see an image with light going through two prisms. In it, the light is split into its constituent colours before passing through a prism, but is then recombined into white light before passing through a second prism, and then being split up again.

Apparently, this was done to allow interesting displays in record shops. Nevertheless, it illustrates one of Isaac Newton’s earliest contributions to optical physics, as it shows how white light is dispersed into its constituent colours by a prism, and how it can be recombined through another prism. A previous Physics World article – “Web of confusion” (May 2022) – has already highlighted a lively classroom discussion on some of the errors therein.

But there is another aspect of the album’s artwork that is equally worthy of attention in physics classrooms. Can we use it determine the refractive index (RI) of the prism illustrated in the original image, and to find out if it corresponds to any available material? The RI of a material is essentially a measure of the extent to which light refracts as it enters or leaves the material. It is easily calculated, if one can measure the angles between the path of the light and a line drawn at right angles to the surface, known as the “normal”.

I printed a few copies of the artwork and enlisted the help of some students to determine the RI of the material on the cover. We added in the normal where the light strikes the prism and where it emerges, and carefully measured the various angles of incidence and refraction, which allowed 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 RI isn’t a problem in itself. After all, at the two extremes of the spectrum, the RI for the violet light has to be greater than that of the red light. That’s why 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. In fact, I had checked out typical values in advance and knew that the RI for red light passing through glass is usually about 1.51, while for violet light it’s 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 on the album cover, the angles of incidence and refraction for the violet light yield an RI of 2.42, which is far too high to be ordinary glass. After digging around, we did find that it closely matches the RI of zincite – a transparent mineral that mainly contains zinc oxide. But zincite is usually tinted either yellow or red, so it hardly matches the image in the photo.

That doesn’t really matter, though, as the material simply cannot be zincite or anything else for that matter. Because if it were zincite, we’d expect a similar, though slightly smaller, value for the red light. In fact, we get a value of 1.15 for red, which doesn’t correspond to any common material that I can track down.

I wondered briefly if variations in the density of the prism could account for the inconsistencies, but that doesn’t work either

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 either. Simply put, if the density (and the RIs) of the glass were varying, we’d expect to see the light follow a curved path through the glass, which does not make any sense. It’s almost as though Storm Thorgerson, who designed the album cover, decided to completely ignore Snell and the laws of refraction.

It wouldn’t even have been that difficult for Thorgerson to create a more accurate and realistic version of the path the light should take. Just look at the image above, which was taken in 2017 by Mason Maxwell – an amateur photographer – using a glass prism. It shows what the Pink Floyd cover should really have been.

Perhaps we can attribute the errors to artistic licence and a lack of general optics expertise. Maybe the request from Pink Floyd keyboardist Richard Wright for a “simple and bold” design – symbolizing the album’s deep themes surrounding riches, greed and conflict – ultimately “eclipsed” scientific accuracy. Either way, 50 years on, this iconic image is here to stay.

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