From the documentary Squaring the Circle (The Story of Hipgnosis), Aubrey ‘Po’ Powell and George Hardie on the cover art for Pink Floyd’s Dark Side of the Moon:
Aubrey Powell: One day, I was looking through this physics magazine about light and about refraction of light. And Storm [Thorgerson] was hovering over me. He looked at me and he said, ‘I’ve got it, Po.’ I said, ‘What? What is it?’ And he said, ‘It’s a prism.’
George Hardie: The reason that it looks like that is that I’ve got to get it from a school exercise book. Although there are plenty of really quite good photographs of prisms with rainbows in them, they don’t have that iconic look.