#### Term Parabola

Term definition

A curve that has the algebraic form
$y=a{x}^{2}$,

where “a” is a constant.

Term etymology

1579, from Gk. parabole "parabola, application," so called by Apollonius of Perga c.210 B.C.E. because it is produced by "application" of a given area to a given straight line.

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#### Term Parabolic

Term definition

A surface that has the shape of a parabola. For a reflective surface, parallel rays striking the surface are reflected and focused at the parabola’s focal point.

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#### Term Parallax

Term definition

The difference in the apparent position of an object when viewed along two different lines of sight. Humans utilize the parallax between their eyes to determine depth.

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#### Term Pechan prism

Term definition

A prism made up of two air-spaced components. It has the ability to revert or invert an image depending on how it is placed. It can be used in convergent, divergent, and parallel light. It may be rotated to compensate for image rotation.

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#### Term Pellin-Broca prism (constant deviation prism)

Term definition

A dispersing prism, often used in monochromators (a device that transmits a narrow range of wavelengths). It is composed of a common right-angle prism with 30° dispersing prisms attached to its perpendicular faces. The spectrum formed is the same as if the two 30° prisms were joined together to form a single 60° prism, but turned through a right angle.

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#### Term Penta prism

Term definition

A five-sided prism containing two reflecting surfaces at 45° to each other, and two refracting faces perpendicular to the entering and emerging beams. The deviation angle of 90° is independent of any rotation of the prism about an axis parallel to the line of intersection of the two reflecting faces. Some common uses are an end-mirror in a rangefinder and an "optical square'' in surveying and alignment machinery.

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#### Term Periodic

Term definition

Repeating in time at regular intervals.

Term etymology

1642, from Fr. périodique (14c.), from L. periodicus, from periodus.

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#### Term Periodic wave

Term definition

A wave of energy in which each point of the wave is repeatedly displaced at equal time intervals.

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#### Term Periscope

Term definition

An optical instrument designed to displace the line of sight in a vertical direction. Used often in submarines.

Term etymology

1899, formed in Eng. from peri- "around" + -scope "instrument for viewing." Earlier (1865) a technical term in photography.

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#### Term Perturb

Term definition

To cause a change in a physical system.

Term etymology

c.1374 (implied in perturbation), probably via O.Fr. perturber (14c.), from L. perturbare "to confuse, disorder, disturb," from per- "through" + turbare "disturb, confuse," from turba "turmoil, crowd."

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#### Term Phase

Term definition

A particular stage in a periodic process or phenomenon. The fraction of a complete cycle elapsed as measured from a specified reference point and often expressed as an angle.

Term etymology

1812, "phase of the moon," back-formed from Mod.L. phases, pl. of phasis, from Gk. Phasis "appearance" (of a star), "phase" (of the moon), from stem of phainein "to show, to make appear." L. sing. phasis was used in Eng. from 1660. Non-lunar application is first attested 1841.

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#### Term Phase difference

Term definition

The difference in phase for two waves with the same frequency.

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#### Term Phase-contrast microscope

Term definition

A microscope that images an object’s variations of phase in as variations in brightness. Direct light passing through a transparent object falls on a ring where it is retarded by a quarter-wave and reduced in intensity. Light diffracted by the phase differences in the transparent object because of variations in refractive index and/or thickness, misses the retarding ring and interferes with the direct light in the manner of an interference microscope. This results in variations in brightness of the image.

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#### Term Photocopy

Term definition

To make a photographic reproduction of printed matter usually using xerography.

Term etymology

(v.) - 1924 in the sense of "make a photographic reproduction," from photo- "light" + copy (q.v.). The usual modern meaning arose 1942 with the advent of xerography. The noun is recorded from 1934. Photostat (1911) was a type of copying machine (trademark Commercial Camera Company, Providence, R.I.) whose name became a generic noun and verb (1914) for "photocopy."

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#### Term Photoelectric effect

Term definition

A phenomenon discovered by Einstein in the early 20th century in which electrons are ejected from a solid when impinged upon by electromagnetic radiation. This led to the understanding of light as particles, or photons. The energy required to strip an electron from an atom is called the ionization energy.

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#### Term Photograph

Term definition

An image created by light from an object striking a light sensitive surface such as a CCD (Charge-Coupled Device) or CMOS (Complementary Metal-Oxide Semiconductor) for electronic images, or photographic film.

Term etymology

(n.) - 1839, "picture obtained by photography," coined by Sir John Herschel from photo- + - graph "instrument for recording." The verb and photography also are first attested 1839, all from a paper read before the Royal Society on March 14, 1839.

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#### Term Photography

Term definition

The process of recording of an image on a light sensitive medium.

Term etymology

1839, from Gk. phos, phot- 'light' and graphos 'writing'. Lit., photography means 'light recording'.

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#### Term Photon

Term definition

An elementary particle of electromagnetic radiation. A quantum of electromagnetic energy of a single mode having a single frequency, polarization, and direction.

Term etymology

"unit of electromagnetic radiation," 1926, from photo- "light" + on "unit."

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#### Term Photonics

Term definition

The science and technology of generating and harnessing light. This includes the science of emission, transmission, modulation, signal processing, amplification, and detection, of electromagnetic radiation in the ultraviolet, visible, and infrared regions.

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#### Term Physical optics

Term definition

The branch of optics that studies interference, diffraction, polarization and other phenomena for which the ray approximation of geometric optics is not valid.

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#### Term Pinhole camera

Term definition

A camera that is essentially a light proof box with a small hole (aperture) on one side. The light from a scene goes through the aperture and creates an inverted image on the other side.

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#### Term Plane

Term definition

A surface that has no curvature.

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#### Term Planoconcave lens

Term definition

A lens with one plane surface and one concave surface.

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#### Term Plasma Channel

Term definition

Plasma channels are formed when a high-powered laser operates at a certain frequency, providing enough energy for an atmospheric gas to break into its ions. A plasma channel has a low electrical resistance and, once formed, will permit continuous current flow if the energy source that heats the plasma can be maintained.

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#### Term Polarization

Term definition

A property of waves that can oscillate in more than one direction. For electromagnetic radiation, the polarization is in the direction of the electric field vector over one oscillation. The polarization vector is usually perpendicular to the direction of propagation.

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#### Term Polarize

Term definition

To align in a certain direction.

Term etymology

1811, from Fr. polariser, coined by Fr. physicist Étienne-Louis Malus (1775-1812) as a term in optics.

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#### Term Polarized light

Term definition

Light in which the electric and magnetic fields each vibrate in a single plane.

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#### Term Polarizer

Term definition

An optical filter that allows light polarized in a specific direction to pass through and blocks light polarized in other directions.

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#### Term Polishing

Term definition

1) To make smooth and glossy. To burnish. 2) The optical process, following grinding, that puts a highly finished, smooth surface on a lens or mirror.

Term etymology

(v.) - c.1300, from O.Fr. poliss-, prp. stem of polir "to polish," from L. polire "to polish, make smooth," of unknown origin. The notion of "to free from coarseness, to refine" first recorded 1340.

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#### Term Porro prism

Term definition

A 45°-90°-45° reflecting prism whose surfaces form the 90° angle reflecting the light beam through a total angle of 180°. Two of these prisms are used in succession in a prism binocular. For each set of prisms as shown below, the image in both inverted and reverted.

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#### Term Power

Term definition

1) The rate at which energy is transferred, used, or transformed. 2) With respect to a lens, it is the reciprocal of the focal length.

Term etymology

(n.) - 1297, from Anglo-Fr. pouair, O.Fr. povoir, noun use of the infinitive in O.Fr., "to be able," earlier podir (842), from V.L. *potere, from L. potis "powerful."

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#### Term Principal plane

Term definition

The surface in a lens or lens system at which the projections of entering and exiting rays intersect.

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#### Term Principal ray

Term definition

Also called chief ray. The ray that passes through the center of an aperture stop in an optical system.

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#### Term Prism

Term definition

A transparent optical element having at least two polished plane faces inclined relative to each other, from which light is reflected or through which light is refracted.

Term etymology

1570, a type of solid figure, from L.L. prisma (Martianus Capella), from Gk. prisma (Euclid), lit. "something sawed," from prizein "to saw." Meaning in optics is first attested 1612.

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#### Term Pupil

Term definition

A hole (aperture) in the center of the iris (aperture stop) in the eye. The pupil allows light to enter the retina.

Term etymology

"center of the eye," 1670 (in L. form from 1398), from O.Fr. pupille (14c.)

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