## 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!

#### Gain

General Terms

The increase in a signal that is transmitted from one point to another.

#### Gain Medium

General Terms

A gain medium is either a gas, liquid, or solid that provides means for optical gain, which is generated by stimulated emission on transitions from higher to lower states. It can amplify the power of light.

#### Galilean telescope

General Terms

A refracting telescope that produces an upright image by using a positive (converging) lens for its objective and a negative (diverging) lens for its eyepiece.

#### Gaussian beam

General Terms

A beam of electromagnetic radiation whose wave front is approximately spherical at any point along the beam and whose transverse field intensity over any wave front is a Gaussian function of the distance from the axis of the beam. A Gaussian function is a mathematical function like the one below that expresses the circular cross section amplitude:

$E\left(r\right)=E\left(0\right){e}^{{\left(r/w\right)}^{2}}$,

where E(r) is the amplitude of the electric field at a radial distance r from the center of the beam, E(0) is the amplitude at the center of the beam (usually maximum amplitude), and w is the radial distance at which the amplitude of the electric field is 1/e ≈ 1/2.71828 of its value on the axis. The variable w is usually called the beam width.

#### Geometric optics

General Terms

Geometric optics is also known as ray optics. It is a field of physics that deals with light as if it truly were composed of rays diverging in various directions from the source and are abruptly bent or turned by refraction or reflection.

#### Glan-Foucault prism

General Terms

A polarizing prism formed from calcite that is like the Nicol prism but has the two parts (prisms) separated by a thin amount of air and cut to such angles that most of the extraordinary beam will be transmitted and the ordinary beam totally reflected. Since part of the extraordinary beam will be reflected, the Foucault prism is somewhat less efficient than the Nicol prsim.

#### Glass

General Terms

A noncrystalline, inorganic mixture of various metallic oxides fused by heating with glassifiers such as silica. Silica (SiO2) is a common component of glass.

O.E. glæs, from W.Gmc. *glasam (cf. M.Du. glas, Ger. Glas), from P.Gmc. base *gla-/*gle-, from PIE *gel-/*ghel- "to shine, glitter, be green or yellow," a color word that is the root of words for grey, blue, green, and yellow (cf. O.E. glær "amber," L. glaesum "amber," O.Ir. glass "green, blue, gray," Welsh glas "blue").

#### Goerz prism system

General Terms

This prism system consists of three single prisms. The system inverts and reverts the image. The image is also displaced by an amount depending on the distance between the first and second prisms.

#### Grating

General Terms

1) A framework or latticework having identical parallel rods that are evenly spaced. 2) In optics, grating is any repetitive structure that can separate the light into its constituent wavelengths. Commonplace CDs and DVDs have fine grooves carved on them, which creates a reflective grating. Physicists and astronomers often use diffraction gratings to determine the wavelengths composing the light being viewed.

(n.) - c.1400, from M.L. grata "lattice," from L. cratis "wickerwork."

#### Grating prism

General Terms

A specific, right-angle prism having a transmission grating replicated on its hypotenuse face and used in applications requiring a system that can produce in-line viewing for one wavelength.

#### Grating spectroscope

General Terms

A spectroscope having a diffraction grating for the resolution of light of various wavelengths.

#### Grazing Angles

General Terms

The angle between the laser beam and the surface of reflection, typically defined when the laser beam is nearly parallel to the surface. This angle can be as small as 1 degree or less.

#### Grinding

General Terms

In optics it is the process in optical system manufacturing that gives an element, such as a lens, its required shape.

O.E. grindan, forgrindan "destroy by crushing" (class III strong verb; past tense grand, pp. grunden), from P.Gmc. *grindanan (cf. Du. grenden), related to ground, from PIE *ghrendh-"crushing" (cf. L. frendere "to gnash the teeth," Gk. khondros "corn, grain," Lith. grendu "to scrape, scratch").

#### Gyroscope

General Terms

1) A device used to measure or maintain orientation, for example, latitude, longitude, and altitude. It is based on the properties of angular momentum and usually consists of a spinning wheel or disk that is free to rotate to any orientation. 2) Often a ring laser is used as a gyroscope. Ring lasers consist of two beams of light of different frequency, the same polarization, and traveling in opposite directions around a closed loop. The loop often consists of a solid block of glass-ceramic material with holes drilled the length of all four sides, and mirrors attached to the corners to form a closed path. When the device is stationary, the optical paths traveled by the two beams are identical. Rotation of the cavity causes a difference in path length between the two beams that can be measured. 3) Fiber optic gyroscope (FOG): An instrument to measure angular rotation, based on the principle that the application of force will alter the wavelength of light as it travels in one direction or the other around a coil of optical fiber wound on a drum. The change in rotation causes a change in the interference pattern produced from the end of the coil, which can be measured.

1856, invented and named in Fr. 1852 by Foucault, from Gk. gyros "circle" + skopos "watcher," because the device demonstrates that the earth rotates.