Real Pink Floyd fans know what it is.
The story behind the 1973 Pink Floyd "Dark Side of the Moon" album cover, quote:
"The final front cover design features a triangular prism, with a beam of light passing through from the left side, resulting on a rainbow beam shooting out of the right side. Set to a black background, the prism was also mirrored on the back cover giving the effect of a consistent beam of light that never ends."