Many optical instruments use right -angled prisms to reflect a beam of light through or (By total internal reflection) such as cameras, binoculars, periscope and telescope.
When a ray of light strikes a face of prism perpendicular, it enters the prism without deviation and strikes the hypotenuse at an angle of . Since the angle of incidence is greater than critical angle of the glass which is , the light is totally reflected by the prism through an angle of . Two such prisms are used in periscope. The light is totally reflected by the prism by an angle of . Two such prisms are used in binoculars.
Prism is also used to invert images without changing their size.