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!

F number (f/#)

General Terms

Sometimes called f-stop, f-ratio, focal ratio, or relative aperture. The ratio of the focal length of a lens to the diameter of the entrance pupil.

Fabry-Perot interferometer

General Terms

A multiple beam interferometer, usually consisting of two parallel highly reflective mirrors. The mirrors are parallel to each other so light can bounce back and forth several times. The same device utilizing a plate with two highly reflective surfaces rather than mirrors is called an etalon.

Femtoseconds

General Terms

One billionth of one millionth of a second. For context, a femtosecond is to a second, what a second is to about 32 million years.

For more information on femtoseconds and where they are used, click the link to the Lasers pamphlet.

Fermat's principle

General Terms

Also known as the principle of least time. Light traveling between two points will take the path that requires the least transit time.

Fiber

General Terms

Short for optical fiber, which is a thin filament of drawn or extruded glass or plastic having a central core and a cladding of lower index of refraction to promote internal reflection.

To learn more about fiber optics click on the link to the Fiber Optics pamphlet.

Test under picture...1540, from Fr. fibre, from O.Fr. fibre, from L. fibra "a fiber, filament," of uncertain origin, perhaps related to L. filum "thread," or from root of findere "to split." Fiber optics is from 1956.

Fiber Optic Line

General Terms

A glass or plastic fiber used to transmit information contained in a beam of light.

To learn more about fiber optics click on the link to the Fiber Optics pamphlet.

Field

General Terms

A physical quantity that has a value at each point in space. Fields extend throughout space, contain energy, and describe what force a particle would feel if placed at a location within the field. For example, in classical physics the gravitational field of an object describes the force per unit mass at each location of space; an electric field produced by a charge describes the electric force per positive unit of charge.

(n.) - O.E. feld "plain, open land" (as opposed to woodland), also "a parcel of land marked off and used for pasture or tillage," probably related to O.E. folde "earth, land," from P.Gmc. *felthuz "flat land," from PIE *pel(e)-tu-, from base *pele- "flat, to spread" (cf. L. planus "flat, level," O.C.S. polje "field"). Common W.Gmc. (cf. O.Fris. feld, M.H.G. velt, Ger. Feld), but not found outside it (Sw. fält, Dan. felt are borrowed from Ger.), though Finnish pelto "field" is believed to have been adapted from P.Gmc.

Field of View

General Terms

The maximum area that can be seen through a lens or an optical instrument.

Field stop

General Terms

An aperture located at an image plane of an optical system that determines the size and shape of the image.

Filament

General Terms

A fine or thinly spun thread, fiber, or wire. An electrical filament in an incandescent light bulb can be used to emit light due to its high temperature.

For more information on filaments click the link to the IR Imaging pamphlet.

Filter

General Terms

A device that attenuates particular wavelengths or frequencies while transmitting others with little change.

(n.) - c.1400, from M.L. filtrum "felt," which was used to strain impurities from liquid, from W.Gmc. *filtiz.

Fizeau interferometer

General Terms

A type of interferometer noted for producing narrow multiple-beam interference fringes. Compared with the Twyman-Green interferometer, the Fizeau interferometer has fewer optical components, does not have the large beamsplitter and can be adjusted to a greater accuracy.

Fluorescence

General Terms

Fluorescence is the emission of light by an atom or molecule that has absorbed electromagnetic radiation, usually of a different wavelength (energy). Typically it is a conversion of a shorter wavelength of light (having higher energy) into longer wavelength (having lower energy) by an atom or molecule. LEDs always produce a narrow spectrum of wavelengths. Phosphors coated on the LED can absorb this light and fluoresce to produce longer wavelengths. White light LEDs utilize fluorescence to convert the specific wavelengths that they produce into a spectrum that looks white.

To learn more about fluorescence click on the link to the Solid State Lighting pamphlet.

Focal length (f)

General Terms

1) The distance from a single thin lens or mirror to its focal point. 2) The effective focal length (EFL) is the distance from the principal point of focal point. The back focal length (BFL) is the distance from the vertex of the last lens to the second focal point. The front focal length (FFL) is the distance from the first lens surface to the first focal point.

Focal plane

General Terms

The plane on which the best focus is formed.

Focal point

General Terms

The point on the optical axis of a lens or mirror, to which an incident bundle of parallel rays will converge, or from which they appear to diverge, as after refraction or reflection in an optical system.

Focus

General Terms

1) A point at which rays of light or other radiation converge or from which they appear to diverge, as after refraction or reflection in an optical system: the focus of a lens. Also called focal point. 2) To adjust the eyepiece or objective so that the image is clearly seen.

For more information on focus, click the link to the Lasers pamphlet.

1644, from L. focus "hearth, fireplace," of unknown origin, used in post-classical times for "fire" itself, taken by Kepler (1604) in a mathematical sense for "point of convergence," perhaps on analogy of the burning point of a lens (the purely optical sense of the word may have existed before 1604, but it is not recorded).

Frankford Arsenal prism number 1

General Terms

This prism will revert the image and, at the same time, it will deviate the line of sight by 115 degrees.

Frankford Arsenal prism number 2

General Terms

This prism inverts and reverts the image and deviates it by 60 degrees.

Frankford Arsenal prism number 3

General Terms

This prism deviates the line of sight by 90 degrees in the horizontal plane and through an angle of 45 degrees in an upward direction. An observer standing at a right angle to the line of sight will see an inverted and reverted image

Frankford Arsenal prism number 4

General Terms

In this prism, the line of sight is deviated by 90 degrees in the horizontal plane and by 45 degrees in the vertical plane simultaneously. An observer standing at a right angle to the line of sight will see a reverted image.

Frankford Arsenal prism number 5

General Terms

The line of sight is deviated by 90 degrees in the horizontal plane and by 60 degrees in the vertical plane in this prism. An observer standing at a right angle to the line of sight will see an inverted and reverted image.

Frankford Arsenal prism number 6

General Terms

In this prism, the line of sight is deviated by 90 degrees in the horizontal plane and by 60 degrees in the vertical plane simultaneously. The prism inverts the image.

Frankford Arsenal prism number 7

General Terms

In this prism, the line of sight is deviated by 90 degrees in the horizontal plane and by 45 degrees in the vertical plane simultaneously. An observer standing at a right angle to the line of sight will see a normal image of the target since the prism neither inverts nor reverts the image.

Fraunhoffer diffraction

General Terms

The diffraction pattern of a source observed at an infinite distance from the source. “Far field diffraction.”

Frequency

General Terms

The number of times a vibrating object oscillates (moves back and forth) in one second. Fast movements produce high frequency (for sound this results in a high pitch/ tone), but slow movements mean the frequency (pitch/tone) is low.

For more information on frequency, click on the link to the Echolocation pamphlet.

1531, from L. frequentem (nom. frequens) "crowded, repeated," of uncertain origin. The v. (1477) is from L. frequentare "visit regularly." Frequency (1551) came to be used 1831 in physics for "rate of recurrence," especially of a vibration.

Fresnel diffraction

General Terms

“Near field diffraction.” The diffraction obtained when the source or the observing screen are a finite distance from the diffraction aperture or obstacle.

Fringe

General Terms

An interference band.

1354, from O.Fr. frenge (1316), from V.L. *frimbia, metathesis of L. fimbriæ (pl.) "fibers, threads, fringe," of uncertain origin. Figurative sense of "outer edge, margin," is first recorded 1894.

Fundamental mode

General Terms

The lowest order mode (lowest energy) of a waveguide.

Fused silica

General Terms

Glass made of pure silicon dioxide (Si02).