Most prisms are used to flip images, not to make rainbows
Splitting white light into a rainbow is the most famous thing a prism does, but it is not what prisms are mostly used for. Inside binoculars and single-lens reflex cameras, prisms bounce light around to turn an upside-down image the right way up, doing the job of mirrors with less light lost.
An optical prism is a clear block with flat polished faces, at least one of them angled; a piece with only two parallel faces is just a window. It can be made of glass, acrylic, fluorite or anything transparent at the wavelengths involved. Rainbows appear because glass slows different colours by different amounts. Blue is slowed more than red and so bends more, fanning white light out into a spectrum.
Reflecting prisms rely on total internal reflection: light striking an inner face at a shallow enough angle cannot escape and bounces back almost perfectly. With anti-reflection coatings on the faces where light enters and leaves, glass prisms lose noticeably less light than metal mirrors. The number of bounces decides the result. An odd number produces a mirror-reversed image, while an even number keeps the image the right way round. A Porro prism sends the image back and sideways, a pentaprism with a roof sends it off at an angle, and a corner cube returns light straight back toward its source.
Glue two right-angled prisms together with a thin coating between them and you get a beam-splitter cube. The coating decides what gets divided, whether by colour, polarisation or simple brightness, and the surrounding glass protects that fragile layer and removes stray back-reflections. Leave a tiny air gap instead and a strange effect appears: some light tunnels across into the second prism, and the amount drops exponentially as the gap widens, so a fine screw can tune it across many orders of magnitude.
Still other prisms use crystals that bend light differently depending on its polarisation, separating the two components cleanly with very low losses. Variants such as the Glan-Taylor prism are built to withstand high-power lasers.
Source: Prism (optics)
