PHYSICS DICTIONARY

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Rockwell Hardness Tests

They are tests for measurement of hardness.  A hardness number is determined by the difference in depth of penetration resulting from the application of an initial minor load followed by a larger major load.

Roentgen Equivalent Man (REM)

The dose equivalent is measured in units of REM when the dose is in RADs.

Roentgen

It is defined as the exposure resulting in an ionizing charge of  1esu in 1 cm³ of air at 0 ^oC and 760mm of Hg pressure (corresponding to a mass of 0.001293gms). One roentgen equals 2.58 x 10^-4 C/Kg.

Rolling Friction

When one body rolls over another body, the friction that acts between those bodies is rolling friction.

Rotating Crystal Method

A method used to study crystal structure. In this method a monochromatic beam of X-ray is incident on a rotating single crystal sample. Diffraction maxima occur when the sample orientation relative to the incident beam satisfies Bragg’s law.

Rotation

It is type of motion of body, in which every particle of body describes circular path and moves along concentric circles about the centre of the rotation. Every particle of the body experiences the same angular displacement, angular velocity and angular acceleration as any other particle of the body.

Rotational Spectrum

Rotational spectroscopy is concerned with the measurement of the energies of transitions between quantized rotational states of molecules in the gas phase. Rotational spectroscopy has primarily been used to investigate fundamental aspects of molecular physics.  It is a uniquely precise tool for the determination of molecular structure in gas phase molecules.

Rotatory Motion

Motion of a body is said to be rotatory under following conditions:

i)   Every particle of the body describes circular path and they move along concentric circles about centre of rotation.

ii)  Every particle of body experiences the same angular displacement, angular velocity and angular acceleration as any other particle of the body.

iii)   It can be produced or changed by an unbalanced torque.

Roughing Pump

Pump which exhausts to atmosphere. This type of pump is usually meant to create low vacuum in the closed vessel.

Rutherford

 It is a unit of Radioactivity. 1Roentgen = 10⁶ disintegrations/sec.

Rutherford Scattering

Rutherford scattering, first discovered by Lord Rutherford, refers to the elastic scattering of a heavy charged particle (such as an α-particle) from a nucleus. In his famous scattering experiment, Rutherford bombarded a thin gold foil by α-particles and studied how many of them deflected from their original direction of motion. He noticed that most of the α-particles passed through the foil un-deflected while very few deflected at very large angles. This experiment proved that most of the space in the atom is empty and the positive charge is concentrated into a small space, which we now call nucleus. This pioneering work by Rutherford changed the way the atoms are visualized forever. Rutherford scattering is sometimes referred to as Coulomb scattering since the Coulomb force between the incident particle and the target nucleus is responsible for the interaction.

Rydberg Constant

The Rydberg constant, symbol R_∞ or R_H, named after the Swedish Physicist Johannes Rydberg, is a physical constant  relating to atomic spectra in the science of spectroscopy. The value is given by 1.0973731534 x 10⁷ m^-1. 

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Resonance (Sound)

It is the phenomenon in which if one of the two bodies of the same natural frequency is set into vibration, the other body also vibrates with larger amplitude under the influence of the first body.

Resonance Absorption (Dielectric)

Dispersion arising during transition from full atomic polarization at radio frequencies to negligible atomic polarization at optical frequency is called as resonance absorption.

Resonant Frequency (Electricity)

 It is the frequency of alternating current at which impedance of a series LCR circuit becomes least.

Rest Mass

When the object has zero speed, it is said to be at rest (as observed by us or by someone accompanying the object). The mass possessed by body at rest is said to be rest mass. It is an intrinsic property of object.

Restoring Force

 The force which acts on the particle in a direction opposite to direction of its displacement.

Resultant Force

 The final force which acts on an object when multiple forces acts on the body.

Reverberation Period

When after the principal sound stops, the intensity of sound remains one millionth part (or 10^{-6 th} part) of its initial intensity i.e., the loudness of sound decreases by 60db, is known as reverberation period.

Reverberation

 The persistence of sound waves even after source stopped emitting is known as “Reverberation”. The phenomenon normally occurs in closed volumes.

Reverse Bias

 Biasing of PN diode in which p-portion is kept at less potential in comparison to n-type. The diode under this bias is poorly electrical conductive.

Reverse Biased Diode

 Configuration of P-N diode in which ‘P’ junction is applied low potential and ‘N’ junction is applied high potential.

Reversible Cell

Cell which can be brought back to original condition after cell is discharged by passing a current through it in reverse direction.

Reversible Change

Reversible change is a change in which if a process is reversed by small changes in external conditions, there is an exact repetition of all changes in reverse path as they were in direct path, and there is no change in state of surroundings and the system taking part in the process.

Conditions of reversibility:

i)      There must be complete absence of dissipation forces such as friction, viscosity etc.

ii)     The direct & reverse process must take place infinitely slowly.  

iii)   The temperature of system must not differ appreciably from its surroundings.

Reversible Process

 If a process is carried out in such a way that every moment the system departs only infitesimally from an equilibrium state, then that process is called reversible process. At every instance, the system remains virtually in state of equilibrium.

Reynolds Number

Reynolds number, in fluid mechanics, a criterion of whether fluid (liquid or gas) flow is absolutely steady (streamlined, or laminar) or on the average steady with small unsteady fluctuations (turbulent).

The Reynolds number, abbreviated N_Re or R_e, has no dimensions. The concept was introduced by George Gabriel Stokes in 1851, but the Reynolds number is named after Osborne Reynolds (1842–1912), who popularized its use in 1883. 

Rheostat

Mechanical arrangement containing continuous coiling of wire at one end and a wiper to decide magnitude of resistance. It is variable resistance used for various applications like regulating currents, check load & line regulations etc.

Richardson-Dushman Equation

Owen W. Richardson in 1928 received the noble prize in physics for his work and the discovery of law governing the phenomenon thermionic emission. Dushman demonstrated modern form of law governing the thermionic emission and law is now referred as Richardson- Dushman equation. It expresses the relationship between the current density ‘j’ in ampere per meter of electrons emitted from metal and absolute temperature ‘T’ in Kelvin of metal as    j=AT² e^{-eØ /kT}

Where ‘A’ is Richardson constant with a theoretical value of 1.2×10⁶ A/m²/K², and eØ is work function of metal.

Rigid Body

It is a body in which the distances between the particles do not change as the body moves about. Rigid body is an idealized concept, there is nothing really rigid body.                                           

Ripple

AC component superimposes over DC

Rocket

Space craft which move with velocity enough to escape earth’s gravitational field with the help of its engines exhaust which gives thrust. Rocket engine works on action and reaction principle. They are used for carrying man made satellites in to space and put in required orbits around earth. 

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Reflectivity

Represents fraction of incident light that is reflected at the interface.

Refraction

Light that is transmitted into the interior of transparent material experiences a decrease in velocity, and as a result it is bent at the interface. This phenomenon is termed as refraction.

Refractive Index

The ratio of light velocity in a vacuum to its velocity in transmitting medium is called as refractive index of the medium.

Refractometer

Device designed by Rayleigh, by which the refractive index of a gas at any pressure can be determined.

Refrigerant

Liquid which on evaporation produces cooling.

Refrigerator law

Spontaneous process cannot be reversed on its own. No refrigerator will have infinite coefficient of performance.

Refrigerator

 It is a device which absorbs heat from a sink (cold body) and rejects heat at source (hot body) when some work is done on it by external source of energy.

Regression

Regression analysis is perhaps the most widely used technique to draw inferences from experimental data. The basic idea behind it is to fit a function that closely represents the trend in the data. The function can then be used to make predictions about the variables involved.

Relative Density

See Specific Gravity

Relative Emittance

The ratio of radiant energy emitted by a body (surface) at a temperature to the radiant energy emitted by the same area of an ideal black body in the same time at same temperature is called as relative emittance of that body (or surface).

Relative Error

The ratio of absolute error to the true value is called as relative error.

Relative Humidity

There is an upper limit for the amount of humidity which air can hold at a given temperature, beyond which saturation occurs. Relative humidity tells you what percentage of this maximum amount of humidity is present in air.

Relative humidity, RH, is the ratio of the actual water vapor pressure to the vapor pressure of saturated air at the same temperature expressed as a percentage. Relative humidity is a relative measure, because the moisture-holding capacity of air increases as air is warmed.

Relativistic Physics

Extends range of application of physical laws to large velocities ‘V’ of order of speed of light in vacuum, includes classical laws when V<<C  and introduces ‘C’ as universal physical constant.

Remanence

For a ferromagnetic or ferrimagnetic material, the magnitude of residual flux density that remains when a magnetic field is removed.

Residual Stress

A stress that persists in a material that is free of external forces or temperature gradients.

Resilience

Capacity of material to absorb energy when it is deformed elastically and then upon unloading, to have this energy recovered is called as resilience.

Resistivity

 It is the reciprocal of electrical conductivity and a measure of a materials resistance to the passage of electrical current.

Resolving Limit of Microscope

It is that minimum distance between two objects kept nearby, the images of which are just resolved by objective of microscope.

or

The smallest angle subtended by the two objects at the objective of telescope, when they are just resolved is called resolving limit of telescope.

Resolving Power of Grating

It is defined as capacity to separate diffraction maxima of two wavelengths which are close to each other.

Resolving Power of Microscope

 It is reciprocal of its resolving limit.

Resolving Power of Telescope

 It is defined as inverse of smallest angle subtended by the two objects at the objective of telescope.

Resolving Power (Prism)

 The ability to form two separate spectral lines of two close wavelengths by a prism is called its resolving power.

Resolving Power

 Ability of an optical instrument to distinguish the images of two very close objects is called the resolving power of optical instrument. 

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Rayleigh’s Criterion of Resolution

According to this criterion, two point sources are resolvable by an optical instrument when central maximum in diffraction pattern of one falls over first minimum in diffraction pattern of other and vice versa.

Rayleigh’s Criterion

Lord Rayleigh put forward a concept for limit of resolution, called Rayleigh’s criterion. According to Rayleigh’s criterion, two close point objects or the two close spectral lines of equal intensities are said to be just resolved by an optical instrument when the principal maxima of diffraction pattern of one coincides with the first minima of diffraction pattern of the other.

Rayleigh’s Jeans Law (Thermal Radiation)

 The radiation emitted by black body is proportional to absolute temperature and inversely proportional to fourth power of wavelength emitted by body.

Rayleigh’s Scattering

In the Rayleigh scattering although there is no resultant change in energy state of molecular system, the system still participates directly in scattering act. This process causes one photon of incident radiation to be annihilated and a photon of same energy to be created simultaneously. The intensity of Rayleigh lines is generally about 10^-3   of the intensity of incident line.

Reactance

It is property of electrical circuit by which it opposes passage of alternating current through it.

Reaction Rate

The reaction rate, R, is the number of reactions per second per cubic centimeter of material.

Reactive Sputtering

Cathodic sputtering in which chemical reactions are involved is termed as reactive sputtering.

Reactor Coolant

Fluid used to remove heat from reactor core produced by Nuclear Fission and transmit the energy to turbine system in the form of steam. 

Reactor Period

Reactor period is defined as that amount of time, normally in seconds, required for neutron flux (power) to change by a factor of e, or 2.718.

Real Gas

No known gas obey ideal gas conditions and hence term real gas has been coined to distinguish from ideal gas an imaginary concept. Real gas also called Vanderwalls gas as the molecules are assumed to be attracting each other, for which Vanderwall’s forces are responsible.

Real Image

When a beam of light diverging from a point after reflection (or refraction) actually converges to a second point, then second point is called the real image of first point. It can be formed on screen.

Reciprocity Theorem (Radiation Physics)

Reversing the positions of a point detector and a point source within an infinite homogeneous medium does not change the amount of radiation detected.

Reciprocity Theorem

Network theorem applicable as per which, current source & voltage are interchangeable. This theorem is applicable to the electrical circuit which has single emf.

Recrystallization

Dissolve the existing crystal and subsequently crystallize a substance from the solution, to form a purified crystal.

Rectifier

Device used to convert alternating current to direct current.

Rectilinear Propagation of Light

Light always travels in straight lines in a homogeneous medium.

Red Shift

It has been observed that some distant nebulae are moving away with a velocity greater than 20x10³ km/sec and the important spectral lines appear to shift towards the red end of the spectrum by 200 A^o. This gives the idea that the universe is expanding.

Reduced Mass

In physics and astronomy, value of a hypothetical mass  introduced to simplify the mathematical description of motion in a vibrating or rotating two-body system. The equations of motion of two mutually interacting bodies can be reduced to a single equation describing the motion of one body in a reference frame centered in the other body. The moving body then behaves as if its mass were the product of the two masses divided by their sum. That quantity is called the reduced mass.

Reduction

The addition of one or more electrons to an atom, ion or molecule is called as reduction.

Reflecting Power

The ratio of amount of radiations reflected by the body in a certain time to the amount of radiations incident on it in the same time is called reflecting power of body.

Reflection

When light radiation passes from one medium into another having different index of refraction, some of the light is scattered at the interface between two media even if both are transparent.