## Pages

### PHYSICS DICTIONARY

<Prev>                   <Next>

Fabry Perot Interferometer

An interferometer made of a transparent plate with two reflecting surfaces or two parallel highly reflecting mirrors. It is named after Charles Fabry & Alfred Perot. It makes use of multiple reflections between two closely spaced partially silvered surfaces. Part of light is transmitted each time the light reaches second surface, resulting in multiple offset beams which can interfere with each other. The large number of interfering rays produces an interferometer with extremely high resolution, somewhat like multiple slits of all diffraction grating increase its resolution.

Face Centered Cubic structure(FCC)

The crystal structure found for many metals has a unit cell of cubic geometry, with atoms located at each of the corners and the centers of all cubic faces.

Fahrenheit Scale

Temperature scale introduced by German physicist Daniel Gabriel Fahrenheit in 1724. On this scale, the freezing point of water is 32 degree Fahrenheit and boiling point 212 degree Fahrenheit at standard atmospheric pressure. This puts boiling and freezing points of water exactly 180o apart. Therefore a degree on Fahrenheit scale is 1/180 of interval between freezing and boiling point. A temperature interval of 1oF is equal to an interval of 5/9 oC.

It is SI unit for capacitance. It is named after English Physicist Michael Faraday. A capacitor is said to have one Farad value when one coulomb of stored charge causes a potential difference of 1 Volt across its terminals.

1Farad = 1 Coulomb / Volt.

The rotation of plane of polarization of a light beam when plane polarized light is passed through a substance in a magnetic field; the direction of travelling of light is parallel to magnetic lines of force. For a given substance, the rotation is proportional to thickness traversed by light and to the magnetic field strength.

The total charge carried by 1 mole of electrons is given by   96487 Cmol-1. The quantity is known as Faraday constant.

The quantitative relationships between electricity and chemical change where first described by Michael faraday in 1830. These are

i) The mass of a chemical substance involved at an electrode is directly proportional to quantity of electricity passed through the cell.

ii)The mass of different substances produced by a given quantity of electricity are proportional to equivalent mass of substances.

Mathematical representation

a) m α Q     b)  m α M/ve

where ‘m’ represents mass of substance deposited at cathode; ‘M’ represents molar mass of substance; ve denotes stoichometric number of electrons in reaction involving reduction/oxidation of chemical species at respective electrode of electrolytic cell.

It states that induced emf in a closed circuit is equal to negative rate at which magnetic flux through it changes.

Fast Neutrons

Neutrons possessing high kinetic energies in the million electron volt range are called fast neutrons.

Fathom(ftm)

It is a unit of length used especially for measuring depth of water. 1 ftm = 1.8288 meters.

Fault

Places where rock structures have broken apart and can slide past each other are called faults.

Feedback

The process of injecting a portion of output energy of some device to the input is known as feedback.

Femto

It is a prefix in the metric system denoting a factor of 10-15.

Fermat’s Principle of Extremum Path

Fermat, in 1658, postulated that “a ray of light in passing from one point to another by any number of reflections or refractions chooses a path along which the time taken is least or extremum.

Fermi Dirac Statistics

Statistics proposed by Fermi & Dirac. The postulates are as follows:

i) Applicable to identical & indistinguishable particles with spin odd multiple of half, such as electrons, protons, neutrons etc, which are called the Fermions;

ii) The minimum volume of a phase cell is h3, so the number of phase cells cannot be increased.

iii) No new microstate is obtained by the interchange of position of particles in a cell.

iv)There cannot be more than one particle in a cell i.e. Pauli’s exclusion principle is obeyed.