#### Gain

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

#### Gain Medium

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

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

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

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

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

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

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

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

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

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

#### Grazing Angles

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.