A pentaprism is a five-sided reflecting prism

Prism (optics)

In optics, a prism is a transparent optical element with flat, polished surfaces that refract light. The exact angles between the surfaces depend on the application. The traditional geometrical shape is that of a triangular prism with a triangular base and rectangular sides, and in colloquial use...

 used to deviate a beam of light

Light

Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

 by 90°. The beam reflects inside the prism twice, allowing the transmission of an image through a right angle without inverting it (that is, without changing the image's handedness

Chirality (electromagnetism)

The term chiral describes an object, especially a molecule, which has or produces a non-superimposeable mirror image of itself. In chemistry, such a molecule is called an enantiomer or is said to exhibit chirality or enantiomerism...

) as an ordinary right-angle prism or mirror would.

The reflections inside the prism are not caused by total internal reflection

Total internal reflection

Total internal reflection is an optical phenomenon that happens when a ray of light strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface. If the refractive index is lower on the other side of the boundary and the incident angle is...

, since the beams are incident at an angle less than the critical angle

Critical angle

Critical angle can refer to:*Critical angle the angle of incidence above which total internal reflection occurs*Critical angle of attack, in aerodynamics; the angle of attack which produces the maximum lift coefficient...

 (the minimum angle for total internal reflection). Instead, the two faces are coated to provide mirror

Mirror

A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...

 surfaces. The two opposite transmitting faces are often coated with an antireflection coating

Optical coating

An optical coating is one or more thin layers of material deposited on an optical component such as a lens or mirror, which alters the way in which the optic reflects and transmits light. One type of optical coating is an antireflection coating, which reduces unwanted reflections from surfaces, and...

 to reduce spurious reflections. The fifth face of the prism is not used optically but truncates what would otherwise be an awkward angle joining the two mirrored faces.

Roof pentaprism

A variant of this prism is the roof pentaprism which is commonly used in the viewfinder of single-lens reflex camera

Single-lens reflex camera

A single-lens reflex camera is a camera that typically uses a semi-automatic moving mirror system that permits the photographer to see exactly what will be captured by the film or digital imaging system, as opposed to pre-SLR cameras where the view through the viewfinder could be significantly...

s. In this case, the image needs to be reflected left-to-right, as the prism transmits the image formed on the camera

Camera

A camera is a device that records and stores images. These images may be still photographs or moving images such as videos or movies. The term camera comes from the camera obscura , an early mechanism for projecting images...

's focusing screen

Focusing screen

A focusing screen is a flat translucent material, usually ground glass, found in a system camera that allows the user of the camera to preview the framed image in a viewfinder. Often, focusing screens are available in variants with different etched markings for various purposes...

, which is itself reflected by the reflex mirror in the camera body. This reflection is done by replacing one of the reflective faces of a normal pentaprism with a "roof

Roof prism

A roof prism is in general any kind of reflective optical prism containing a section where two faces meet at a 90° angle...

" section, with two additional surfaces angled towards each other and meeting at a 90° angle. This form of the prism changes the handedness of the image.

Pentaprism in surveying

In surveying

Surveying

See Also: Public Land Survey SystemSurveying or land surveying is the technique, profession, and science of accurately determining the terrestrial or three-dimensional position of points and the distances and angles between them...

 a double pentaprism (two pentaprisms stacked on top of each other) and a plumb-bob

Plumb-bob

A plumb-bob or a plummet is a weight, usually with a pointed tip on the bottom, that is suspended from a string and used as a vertical reference line, or plumb-line....

 are used to stake out right angle

Right angle

In geometry and trigonometry, a right angle is an angle that bisects the angle formed by two halves of a straight line. More precisely, if a ray is placed so that its endpoint is on a line and the adjacent angles are equal, then they are right angles...

s, e.g. on a construction

Construction

In the fields of architecture and civil engineering, construction is a process that consists of the building or assembling of infrastructure. Far from being a single activity, large scale construction is a feat of human multitasking...

site.