Optics Dictionary

Sometimes reading a scientific explanation is as difficult as reading Parseltongue. This section features definitions and etymology for the terms and phrases you will encounter as you explore the science of light. Etymology is the study of the history of words — when they entered a language, from what source, and how their form and meaning have changed over time. Ever wonder how the word optics got its meaning? OK — probably not but now you can find out!


General Terms

A light-tight box that receives light from an object or scene and focuses it to form an image on a light-sensitive material or a detector.

16c., in Mod.L. camera obscura "dark chamber" (a black box with a lens that could project images of external objects), from L. camera "vaulted room," from Gk. kamara "vaulted chamber," from PIE base *kam- "to arch." Contrasted with camera lucida (L., "light chamber"), which uses prisms to produce a traceable image on paper beneath the instrument. Shortened to camera when modern photography began in 1840.

Carl Zeiss prism system

General Terms

A system consisting of three single prisms that invert and revert the image, but do not deviate the line of sight. The line of sight is displaced by an amount depending on the distance between the first and second prisms.

Cassegrain Telescope

General Terms

A two-mirror design used in astronomical telescopes. The primary mirror is concave, parabolic and has a hole in its center to allow light to pass. The secondary is a smaller hyperbolic, convex mirror. The image formed is located behind the vertex of the primary. There are many variations of this system.

Cathode Ray Tube

General Terms

A vacuum tube with an electron gun at one end and a fluorescent screen at the other. Electrons emitted from a heated filament are accelerated by a series of annular anodes at progressively higher positive voltages. These emitted electrons form a beam, which is deflected by two pairs of electrostatically charged plates or electromagnets located between the gun and the screen. Upon striking the screen, coated with phosphor (often with red, green, and blue zones), fluorescence occurs emitting light.

Catoptric system

General Terms

An optical system that only uses mirrors for image formation.

From G.k katoptrikos, from katoptron, "mirror." Of or relating to mirrors and reflected images.

Catadioptric system

General Terms

An optical system that uses mirrors and lenses for image formation.


General Terms

1) The hollow of a body. 2) An optical resonator whose resonant modes can be used in laser systems. 3) A device used to select waves of a particular frequency.

1541, from M.Fr. cavité, from L.L. cavitas "hollowness," from L. cavus "hollow."

Center of curvature

General Terms

The center of the sphere whose surface can be used to describe the location and curvature of the surface of a lens or mirror.

Chief ray

General Terms

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


General Terms

A small fiber optic endoscope used in laser surgery.


General Terms

An instrument designed to analyze the optical effects of color.

1597 (of music), 1831 (of color), from Gk. chromatikos "suited for color," from chroma (gen. chromatos) "color, complexion, character," but also used of music, orig. "skin, surface."


General Terms

Having color.

Chromatic aberration

General Terms

A lens aberration resulting from the normal increase in refractive index of all common materials toward the blue end of the spectrum.


General Terms

The low-refractive-index material that surrounds the core of an optical fiber, whose purpose is to contain core light while protecting against surface contaminant scattering.

1300, alternate pt. and pp. of clothe, from O.E. geclæ!d, pp. of clæ!an "to clothe," from cla" "cloth." The covering of the core in an optical fiber, 1956.

Clear aperture

General Terms

The opening of an optical system, or its components, which limits the extent of the bundle of rays on the given surface. It is usually circular and specified by its diameter.


General Terms

Properties of correlation between a single wave or multiple waves. The more one is able to predict a second wave based on information of the first wave, the more correlated and the more coherent the two waves are.

c.1555, from M.Fr. cohérent, from L. cohærentem (nom. cohærens), prp. of cohærere "cohere," from com- "together" + hærere "to stick."

Coherence length

General Terms

The length over which waves maintain a specified degree of coherence.

Coherent beam

General Terms

A beam of light whose photons all have the same wavelength, phase, and direction.

To learn more about coherent beams visit the Lasers pamphlet.

Cold mirror

General Terms

A mirror whose coating reflects visible radiation and transmits the infrared radiation.


General Terms

1) to make parallel, to line up. 2) The process by which a divergent beam of radiation or particles is converted into a parallel beam.

From L.L. collimare, collimat-, alteration of L. collineare "to aim" and/or com- + L. lineare," to make straight."

Collimated Light

General Terms

Light whose rays are nearly parallel and therefore will spread slowly as it propagates.

Visit the Laser pamphlet to learn more about collimated light.


General Terms

The hue, saturation, and brightness that is visibly detectable.

c.1225, from O.Fr. colur, from L. color (acc. colorem) "color, hue," from O. L. colos, orig. "a covering" (akin to celare "to hide, conceal"), from PIE base *kel- "hide." O.E. words for "color" were hiw, bleo. The verb is from c.1300, earliest use is figurative.


General Terms

A lens aberration resulting from different magnifications in the various lens zones.


General Terms

A hollow curved surface; curved inward.

14c., from L. concavus "hollow," from com- intensive prefix + cavus "hollow."


General Terms

Characterized by having the same center.

1391, from O.Fr. concentrique, from M.L. concentricus, from com- "together" + centrum "circle, center."


General Terms

A single positive lens or group of lenses in a projection system that collect light and cause it to be evenly distributed.

From M.E. condensen, from O.F. condenser, from L. condensare "reduce volume or compass of."


General Terms

Elements whose focal points coincide.

From L. conducere "to lead together" and L. focus " hearth, fireplace."


General Terms

Light emitted from one element is focused on the other and vice versa.

c.1450, from L. conjugationem, from conjugare "to join together."


General Terms

The apparent difference in brightness between light and dark areas of an image.


General Terms

A way for a musician to modify the musical instrument to affect the sound it produces. Musical instruments including the voice, allow musicians to change pitch, loudness, even the tone color to some degree. For example, a trumpet can play the same note loud, brassy, and bright, or softer with a mellow tone.

To learn more, click on the link to the Acoustics pamphlet, which discusses sound waves and music.


General Terms

1) Comes together from different directions. 2) The bending of light rays toward each other.


General Terms

Spherically shaped; curves outward.

1571, from M.Fr. convexe, from L. convexus "vaulted, arched," pp. of convehere "to bring together," from com- "together" + vehere "to bring." Possibly from the idea of vaults carried together to meet at the point of a roof.


General Terms

The light-conduction portion of an optical fiber, defined by the region of high refractive index.

1398, probably from O.Fr. coeur "core of fruit, heart of lettuce," lit. "heart," from L. cor "heart." Center of an optical fiber, 1956.

Corner cube prism

General Terms

Corner cube prism also called corner cube reflector and corner reflector. A prism having three mutually perpendicular surfaces and a hypotenuse face. Light entering through the hypotenuse is reflected by each of the three surfaces in turn and emerges through the hypotenuse face parallel to the entering beam. The prism returns entering beams to the source. This can also be constructed from three first-surface mirrors.

Cornu double prism

General Terms

A compound prism formed by cementing together two 30° prisms, one of right-handed and one of left-handed quartz. It has good ultraviolet-transmitting qualities and no double refraction.

Cornu-Jellet prism

General Terms

A prism formed by dividing a Nicol prism in a plane parallel to the path of vibration of the transmitted light, and taking out the wedge-shaped piece. After the pieces are rejoined, the planes of vibration of the transmitted light of the two halves form a small angle with each other.

Critical angle

General Terms

The smallest angle of incidence at which total internal reflection occurs. When the critical angle is exceeded, the light is totally reflected back into the denser medium. The critical angle will vary with the indices of refraction of the two media:  sin I c = n n MathType@MTEF@5@5@+=feaagCart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLnhiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr4rNCHbGeaGqipu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=xfr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaci4CaiaacMgacaGGUbGaamysamaaBaaaleaacaWGJbaabeaakiabg2da9maalaaabaGabmOBayaafaaabaGaamOBaaaaaaa@3DA1@  and is shown below by "c."

Crossed prisms

General Terms

The positioning of two Nicol prisms so that their axes are at right angles to each other. Using this arrangement, light transmitted by one prism will not be transmitted by the other.


General Terms

The measure of departure from a flat surface, as applied to lenses; the reciprocal of radius.

Early 15c., from L. curvus "crooked, curved, bent."