Porro prism

A Porro prism is an optical device that inverts and reverses an image. It consists of a pair of right-angled prisms. When light enters the first prism, it is totally reflected twice before entering the second prism, where it is reflected twice again. This arrangement allows for the shortening of the physical length of optical instruments like binoculars.1

Porro prisms are a type of reflecting prism, which are glass components with precisely angled plane surfaces that manipulate light through transmission and reflection. They are commonly used in binoculars to correct the image orientation, with two Porro prisms working together to reverse the beam direction both vertically and horizontally. This contrasts with a roof prism, which also changes light direction and reverses an image but performs these functions differently. Porro prisms are essential in many optical viewing instruments, including binoculars and periscopes.234