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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.


Radar is acronym for "Radio Detection And Ranging." A radar system usually operates in the ultra-high-frequency (UHF) or microwave part of the radio-frequency (RF) spectrum, and is used to detect the position and/or movement of objects. Radar can track storm systems, because precipitation reflects electromagnetic fields at certain frequencies. Radar can also render precise maps. Radar systems are widely used in air-traffic control, aircraft navigation, and marine navigation.


It is the angle subtended at the centre of a circle by an arc whose length is equal to the radius of that circle.

1radian = 57o 17’44”; 2π radian=360o

Radiant Energy (Heat)

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

Radiant Energy

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

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. 

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 Length

Radiation length is a physical quantity in nuclear physics which describes overall attenuation of electrons and photons passing through matter. It is defined as thickness of a material that an electron travels such that it loses about 63% of its energy in Bremstrahlung.

Radiative Capture

A particle may combine with a nucleus to produce a new nucleus or a compound nucleus which is in an excited state. The excess energy is emitted in the form of  γ-ray photons. This type of process is known as radiative capture.

Radiator (Thermal)

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

Radio Frequency

Frequency of radio waves or signals in the range of about 3KHz to 300GHz.

Radio Waves

They are type of electromagnetic waves with frequencies ranging from 300GHz to as low as 3 KHz.

Radioactive Decay

When nuclei emit subatomic particles, their configuration, state, and even identity may change. Except for gamma decay, in which nucleus retains its identity, all other decays transform the nucleus into a totally different one. This process is called radioactive decay.

Radioactive Disintegration

 Spontaneous breaking up of nucleus is known as radioactive disintegration.

Radioactive Transformation

The disintegration of an unstable nucleus resulting in another nucleus is called as radioactive transformation.

Radio Waves

Waves having wave length in range of 106 to 10-3 meters are called as radio waves. Sources of radio waves are electrons vibrated by electronic circuits, radio, stars, and galaxies including pulsars and quasars.


Radium was discovered by Marie Curie and her husband Pierre in 1898. Radium is an alkaline earth metal. Radium has a melting point of 700°C, boiling point of 1140°C, specific gravity estimated to be 5, and valence of 2. Pure radium metal is bright white when freshly prepared, although it blackens upon exposure to air. The element decomposes in water. Radium emits alpha, beta, and gamma rays. It produces neutrons when mixed with beryllium. A single gram of Ra-226 decays at the rate of 3.7x1010 disintegrations per second. A gram of radium produces around 0.0001 ml (STP) of radon gas (emanation) per day and about 1000 calories per year. Radium loses about 1% of its activity over 25 years, with lead as its final disintegration product. Radium is a radiological hazard. Stored radium requires ventilation to prevent the build-up of radon gas. 

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 does possess.

Radius Vector

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


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


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

Raman Effect

When a monochromatic beam of light is passed through a gas, liquid or transparent solid body, a small fraction of light is scattered in all directions. The scattered light consists of frequencies greater and smaller than incident beam frequency in additional to original, the phenomenon is known as Raman Effect.

Ramsden Eye Piece

It consists of two plano-convex lenses of same focal lengths with a separation of two third the focal length of either lens. The two lenses are placed with their surfaces facing each other.

Random Errors

Random errors refer to the errors that are statistical in nature.

Rank of a Tensor

The rank of a tensor only indicates the number of indices attached to its per component. The rank of Tensor gives the number of mode of changes of a physical quantity when passing from one system to other which is in rotation relative to the first.

Rayleigh Scattering

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

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