That of “The Dark Side of the Moon” (1973), the masterpiece album by Pink Floyd, is probably the most loved cover by scientists . Yet it contains two errors.
Designed by the graphic designer Storm Thorgerson at the Hipgnosis photographic studio, it schematically reproduces a famous experiment conducted by Isaac Newton in 1676 on the optical phenomena of light refraction and dispersion : through a triangular glass prism, a beam of white light is deflected and broken down into its fundamental components, that is, the seven colors of the rainbow.
The Dark Side of the Moon | Mistakes
And here the first mistake arises . On the album cover, the indigo is missing, so the colors are only six. To be honest, it must be said that it is a mistake that the Hipgnosis studio deliberately made for graphics needs: it had to eliminate the indigo from the cover because too similar to purple. Their combination would not have created enough contrast.
Unwanted, apparently, it is, instead, the second mistake . The decomposition of the light should also take place inside the prism, while in the figure on the cover the beam of light appears colored only at the exit.
The physical phenomenon
The angle of refraction depends on the frequency as well as the propagation speed (except in the vacuum). The different refraction is responsible for the decomposition of the white light by a prism . Prism which, Newton discovered , acts as a “sieve”, separating the colors from each other. Before Newton there were those who thought that white light tinged as a function of the distance traveled in the glass of the prism (or in the water of a drop, in the case of the rainbow).
The famous experiment was conducted in 1676 but his first experiences with Newton prisms began to do them already in 1665, when the plague struck London and the University of Cambridge, forcing the English physicist and mathematician to take refuge in his country house , in Woolsthorpe. It was here that Newton laid the foundations of differential and integral calculus, came up with an embryonic formulation of the theory of universal gravitation. In addition to formulating the foundations of his color theory.