Pentaprism

A five-sided optical prism that redirects a light beam by 90 degrees, largely independent of small changes in its angle of incidence.

Total internal reflection: Reflection at a boundary that occurs when light inside a higher-index medium meets a lower-index medium above the critical angle. It can provide one of the internal reflections that redirect light inside a pentaprism.
Single-lens reflex camera: A camera that uses a movable mirror to direct light from its taking lens into an optical viewfinder. Its viewfinder uses a pentaprism to turn and orient the lens image for viewing.
Refraction: The change in direction of a wave as it crosses between materials where its speed differs. Light refracts at the prism's entry and exit faces, while the internal reflections provide the main directional fold.
Dispersive prism: A prism used to separate light into wavelengths because its refractive index varies with wavelength. A pentaprism is designed chiefly to redirect a beam, not to spread it into a spectrum.
Mirror coating: A reflective material layer deposited on a surface to reflect incident light. Pentaprism reflecting faces are commonly coated because their geometry does not ensure total internal reflection.
Viewfinder: An optical or electronic camera component used to compose and inspect a scene before or during exposure. The pentaprism is the optical element that makes an SLR viewfinder image upright and correctly oriented.
Snell's law: The equation relating the angles of incidence and refraction to the refractive indices of two media. It governs how rays enter and leave the pentaprism's glass.
Porro prism: A pair of right-angle prisms that folds an optical path and erects an image in binoculars and other instruments. Its multiple reflections erect images through a different geometry than the pentaprism's single right-angle deviation.
Law of reflection: The optical rule that a reflected ray leaves a surface at the same angle to its normal as the incident ray. The prism's two reflections obey this rule, while their geometry fixes the net turn.
Focusing screen: A translucent surface on which a camera lens forms an image for viewing and focusing. In an SLR, the screen forms the image that the pentaprism relays to the eyepiece.

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Digital single-lens reflex cameraRelated: It turns the focusing-screen image toward the DSLR viewfinder eyepiece.