#### Term Gain

Term definition

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

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#### Term Gain Medium

Term definition

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.

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#### Term Galilean telescope

Term definition

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.

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#### Term Gaussian beam

Term definition

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.

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#### Term Geometric optics

Term definition

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.

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#### Term Glan-Foucault prism

Term definition

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.

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#### Term Glass

Term definition

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

Term etymology

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").

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#### Term Goerz prism system

Term definition

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.

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#### Term Grating

Term definition

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.

Term etymology

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

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#### Term Grating prism

Term definition

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.

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#### Term Grating spectroscope

Term definition

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

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#### Term Grazing Angles

Term definition

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.

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#### Term Grinding

Term definition

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

Term etymology

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").

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#### Term Gyroscope

Term definition

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.

Term etymology

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

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