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

An endoscopic surgical instrument that includes a channel for the introduction of supplementary instruments.


General Terms

Often called the ‘voice box,’ it is located at the top of the windpipe at the lower end of the throat. It is made up of muscles, membranes and cartilages, and contains the vocal folds (vocal cords).

To learn more, click on the link to the Acoustics Pamphlet.


General Terms

The term LASER is an acronym for Light Amplification by Stimulated Emission of Radiation. It represents an optical source that produces photons (packets of light energy) that are coherent (in phase), collimated (along a straight line), and monochromatic (of the same frequency or wavelength). Lasers consist of a resonant cavity made usually with two reflecting surfaces, a gain medium, and an energy source. The gain medium can be a solid, liquid, or a gas.

To learn more about Lasers, click here.

1960, acronym for "light amplification by stimulated emission of radiation."

Lateral color

General Terms

A lens aberration resulting in image size variation as a function of wavelength.

Law of reflection

General Terms

States that the angle of reflection is equal to the angle of incidence.

See defintion for "angle of incidence" and "angle of reflection."

Law of refraction

General Terms

States that the ratio of the sine of the angle between the normal and refracted ray is a constant. See definition for "angle of refraction."

LED (light emitting diode)

General Terms

A semiconductor device that converts electricity into light efficiently. Light from LED bulbs looks almost identical to light from conventional bulbs.

To learn more about LEDs click on the link to the Solid-State Lighting Pamphlet.

Leman prism

General Terms

This prism inverts and reverts the image while laterally shifting the image by an amount equal to 3A, as shown in the figure.


General Terms

A transparent optical component consisting of one or more pieces of optical glass with surfaces so curved that they serve to converge or diverge the transmitted rays from an object, thus forming a real or virtual image of that object.

1693, from L. lens (gen. lentis) "lentil," on analogy of the double-convex shape.


General Terms

1) The length of time something is in a certain state, such as an electron bound to an atom, in an energy level higher than its ground state. 2) Lifetime is the number of hours a bulb lasts before it burns out. LEDs last for 30,000 to 50,000 hours, which is 50 times longer than incandescent lamps and 5 times longer than fluorescent lamps.

To learn more about this, click on the link the Solid-State Lighting pamphlet.


General Terms

An electromagnetic wave that is visible to the eye. The elementary particle that defines light is the photon. Visible light ranges from 400 to 750 nm in wavelength.

To learn more about this, click on the link the Polarization pamphlet.

(n.) - "brightness," O.E. leht, earlier leoht, from W.Gmc. *leukhtam (cf. O.Fris. liacht, M.Du. lucht, Ger. Licht), from PIE *leuk- "light, brightness" (cf. Skt. rocate "shines;" Arm. Lois "light," lusin "moon;" Gk. leukos "bright, shining, white;" L. lucere "to shine," lux "light," lucidus "clear;" O.C.S. luci "light;" Lith. laukas "pale;" Welsh llug "gleam, glimmer;" O.Ir. loche "lightning," luchair "brightness;" Hittite lukezi "is bright"). The -gh- was an Anglo-Fr. scribal attempt to render the O.E. hard -h- sound, which has since disappeared.

Light dispersion

General Terms

The process by which white light is separated into its component wavelengths. Prisms and gratings are dispersion devices.

Light Waves

General Terms

Light is an electromagnetic wave that is visible to the eye. The elementary particle that defines light is the photon.

To learn more, click on the link to the Spectroscopy pamphlet.


General Terms

Most simply defined as the discharge of electricity from one cloud to another, or from a cloud to the ground.

Lippich prism

General Terms

A small half-shade analyzer placed in the eyepiece of a polarimeter to determine the character of the polarized light emitted by the instrument. It consists of a Nicol prism situated so that it covers half of the eyepiece's field of view.

Liquid crystal

General Terms

A material that possesses less geometrical regularity or order than normal solid crystals, and whose order varies in response to alterations in temperature and other quantities.

Liquid crystal displays

General Terms

A display formed by sandwiching a layer of liquid crystal material between two sheets of glass; a transparent conductive coating on the glass is etched to form character segments. An applied voltage causes the appropriate segments to darken as the liquid crystal changes arrangement.

Local area network (LAN)

General Terms

Data communications network in a clearly defined geographical locatio, and extending no more than a few miles in length. It generally takes in an office building or group of buildings, a campus or the like, offering many nodes and connecting computers and peripherals with high-capacity links.


General Terms

A medium that scatters or absorbs radiation that passes through it.

O.E. los "loss, destruction," from P.Gmc. *losom- (cf. O.N. los "the breaking up of an army"), from PIE *lau-. The modern word, however, probably evolved 14c. from lost, the original pp. of lose.


General Terms

Glowing or giving off light.

"shining, bright, luminous," c.1500, from L. lucentem, prp. of lucere "to shine" (see light (n.)).


General Terms

A unit of measurement equal to the amount of light emitted per second in a unit solid angle of one steradian from a uniform source of one candela.

L. lumen, "an opening, light." A unit of light in the SI system.


General Terms

An object that gives light, such as a celestial object like the Sun or Moon.

c.1450, "source of (artificial) light," from M.Fr. luminarie "lamp, light," from L.L. luminare "light, torch, lamp, heavenly body," lit. "that which gives light," from L. lumen (gen. luminis) "light." Luminosity in astronomy sense of "intrinsic brightness of a heavenly body" (as distinguished from apparent magnitude, which diminishes with distance), is attested from 1906.


General Terms

Relating to light perceived by the eye, rather than the actual energy associated with the electromagnetic radiation.

1432, "full of light," from L. luminosus "shining, full of light," from lumen (gen. luminis) "light," related to lucere "to shine. "


General Terms

Related to the moon.

M.E., "crescent-shaped," from O.Fr. lunaire, from Latin lunaris, "of the moon," from luna, "moon."