A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
 
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Refractive index:

The ratio of light velocity in a vacuum to its velocity in transmitting medium.

Refrigerant:

Liquid which on evaporation produces cooling.

Reversible cell:

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

Plasticity:

The property of a body by virtue of which it cannot regain its

Radiant Energy:

It is defined as energy of particles (excluding rest energy) emitted, transferred or received. (as per ICRU, 1980)

Radiation Exposure:

It is defined by ICRU(1980) as “the quotient of dQ/dm, where the value of dQ is absolute value of total charge of the ions of one sign produced in air when all the electrons liberated by photons in air of mass ‘dm’ are completely stopped in air”. 

Radiation Absorbed Dose:

It is the unit for measure of energy deposited by ionizing radiation per unit mass of material. One RAD is equal to energy absorption of 100 ergs per gram of material. 

Refrigerant:

Liquid which on evaporation produces cooling.

Reversible cell:

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

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.

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 1 esu 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.

RAD (Radiation Absorbed Dose):

It is the unit for measure of energy deposited by ionizing radiation per unit mass of material. One RAD is equal to energy absorption of 100 ergs per gram of material.

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.

Rainbow:

It is a phenomenon due to combination of refraction of sunlight by spherical wavelet droplets behaving like prism (dispersion) and of “Total internal reflection”.

(or)

The spectrum formed due to dispersion of sunlight by water drops settling down after rain.

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.

Rayleigh scattering:

When a scattering occurs with molecules which are smaller than the wavelength of incident radiation is called Rayleigh scattering.

Rutherford:

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

Resonant frequency (electricity):

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

Roughing Pump:

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

Resilience:

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

Reduction:

The addition of one or more electrons to an atom, ion or molecule.

Residual stress:

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

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.

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.

Radiant energy (heat):

The heat energy transferred by the mode of radiation is called radiant energy.

Radiator (thermal):

If the temperature of the body is higher than the temperature on its surroundings, the body emits thermal radiations all around from its surface.

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 relative emittance of that body(or surface).

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.

Rolling friction:

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

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.

Radius vector:

When a particle describes circular path, the line joining centre of circle and position of that particle at any instant is called radius vector.

Restoring force:

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

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.

Reduction:

The addition of one or more electrons to an atom, ion or molecule.

Reverberation:

The persistence of sound waves even after source stopped emitting waves is known as “Reverberation”.

Reciprocity theorem:

According to this theorem, current source & voltage are interchangeable. This theorem is applicable to that network which has single emf.

Statement: For network with only one emf, the current in jth branch when emf acts in ith branch (i#j)is same as current in ith branch when emf acts in jth branch.

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.

(or)

If velocity of a body is reduced it moves towards centre, it is called rigid body.

Radius of gyration:

It is that distance from the axis of rotation where the whole mass of body can be placed to get same moment of inertia which the body actually has.

Reactive sputtering:

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


Rigid body:

A body in which distances between the particles do not change as the body moves about. It should b e noted that rigid body is an idealized concept and there is nothing like a really rigid body.