Possible Clues:

• Solid that, when divided into three parts, describes this puzzle's theme
• Object depicted on the cover of Pink Floyd's 'The Dark Side of the Moon'
• Female character in The Importance of Being Ernest — optical device
• Light-refracting device
• Cover image on Pink Floyd's 'The Dark Side of the Moon'
• Glass splitting white light
• Pointy thing that makes light go off in a new way
• Square, primarily, in proper solid shape
• Beam-splitting crystal
• Object that separates light into a spectrum