PHYSICS DICTIONARY

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Big Bang Theory

The Big Bang theory is an effort to explain what happened at the very beginning of our universe. According to the theory, our universe sprang into existence as "singularity" around 13.7 billion years ago. Our universe is thought to have begun as an infinitesimally small, infinitely hot, infinitely dense, something called as singularity. The Big Bang theory is the scientific theory that is most consistent with observations of the past and present states of the universe, and it is widely accepted within the scientific community. Georges Lemaître first proposed the Big Bang theory which he called as "hypothesis of the primeval atom". The framework for the Big Bang model relies on Albert Einstein's general relativity and on simplifying assumptions such as homogeneity and isotropy of space. 

Bimetallic Strip

Two different metal strips of equal lengths placed on each other is called bimetallic strip. On heating, bimetal bends with material of greater linear expansion to convex side. On cooling, it bends with material of greater linear expansion on concave side.   

Binary System

The pair of stars which orbit around each other is referred to as binary system. The center of mass of the binary system lies in between the two stars. The two stars rotate about this point. 

Binding Energy per Nucleon

See average binding energy.

Binding Energy

The energy equivalent of mass defect is called binding energy. It is this energy which binds nucleons together. Hence it is the energy required for breaking a nucleus into free neutrons and protons.

Binoculars

Binoculars are a parallel combination of two telescopes for viewing an erect 3 dimensional image with both eyes. The image of same size can be viewed with both eyes which is uncomfortable for users to see with single eye. The distance between telescopes is adjustable.

Bioluminescence

It is the phenomenon of emission of light by living creatures because of chemical reaction.

Biophysics

Biophysics is an interdisciplinary science that deals with the application of physics to biological processes and phenomena.

Biot & Savart law

This law gives relation between magnetic field due to current carrying conductor and current flowing in the element. dB at a distance ‘r’ from a current element ‘dL’ carrying a current I is found to be proportional to I , to the length dL and inversely to the square of the distance r. The direction of magnetic field is perpendicular to line element dL as well as radius r.

Biprism

A triangular prism with vertex angle of nearly 180^o used to obtain images of a single source in observing the interference light.

Birefringence

Some crystals have property of splitting incident light ray into two refracted rays. It is due to optical anisotropy in the binding forces between the atoms forming a crystal. They have two indices of refraction. This property is called birefringence.

Black Body

A body which completely absorbs radiation of all wavelengths incident on its surface and doesn’t reflect any part of it is called as black body.

                                                        (or)

It is a body which emits thermal radiations of all wavelengths when heated to high temperature. 

Black Hole

A black hole is a region of space-time of extreme density with such strong gravitational attraction due to which nothing can escape, even light. When a star burns through the last of its fuel, it may find itself collapsing. For smaller stars, up to about three times the sun's mass, the new core will be a neutron star or a white dwarf. But when a larger star collapses, it continues to fall in on itself to create a stellar black hole.

Bloch Theorem 

It is a mathematical theorem which gives us the form of electron wave function in a periodic potential. As per this theorem, electron in a one dimensional lattice behaves as plane wave.

Blue Moon

When two full moons occur in a single month, the second full moon is called a "Blue Moon".

Stefan's -Boltzmann Law

This law states that the total amount of radiant energy emitted by a black body per second per unit area is directly proportional to the fourth power of its absolute temperature i.e. E∝T⁴ or E=𝜎T⁴ where 𝜎 is called Stefan's constant. It has a value of 5.67 x 10⁻⁸ Wm⁻²K⁻⁴. This law is strictly true only when the medium surrounding the black body is vacuum.

The same law was established later by Boltzmann theoretically from thermodynamical considerations. Hence, this law is known as Stefan-Boltzmann law.

Consider the case of black body 'A' at absolute temperature 'T1' which is surrounded by another black body at absolute temperature 'T2'.

Now, 

Heat lost by black body 'A' is 𝜎T1⁴ 
Amount of heat absorbed by black body 'A' from black body 'B' is  𝜎T2⁴.

Therefore, Net amount of heat emitted by body 'A' per second per unit area is 𝜎(T1⁴ - T2⁴).

This is the form of "Stefans Boltzman Law". 

Note: This law is true only when medium surrounding the body is vacuum.

Black body and its Radiation

A perfectly black body is the one which absorbs all the radiations of all wavelengths incident on it. Since it neither reflects not transmits any radiation it appears black in color what may be the color of incident radiation.

According to Kirchoffs law, a body which is capable of absorbing radiation must also be capable of emitting all possible wavelengths. So a perfectly black body is a good absorber as well as good radiator. When it is heated to a suitable high temperature, it emits radiations of all wavelengths (continuous spectrum). As the radiations emitted by black body is rich in maximum possible wavelengths and hence such Radiations are known as full Radiation or Total Radiation.     

The wavelength of emitted Radiation by a black body depends only on its temperature and is independent of the material of the body.

There is no body acting as perfect black body. The nearest approach is lamp black or platinum black. These are capable of absorbing the visible and a part near infrared but far infrared (heat Radiation) are reflected. So perfectly black body is just an ideal concept. For all practical purposes a lamp blacked surface can be considered as perfectly black body.

Energy Distribution in Black body Radiation

The distribution of energy in black body radiation for different wavelengths and at various temperatures was determined experimentally by Lummer and Pringsheim in 1899. They used the black body as an electrically heated chamber with narrow aperture.

The temperature of heated enclosure is measured by thermocouple.

The parallel beam of Radiation is allowed to incident on a "fluorspar prism" instead of a glass prism. The reason behind not using glass prism is that it absorbs some heat radaition.

The radiation is detected by means of Bolometer. Bolometer is an instrument to detect Thermal Radiation. The Bolometer is a linear type due to Lummer and Kurlabaum and is fitted with galvanometer 'G'. The deflection produced in the Galvanometer gives the intensity of Radiation, Eƛ. This is defined such that quantity Eƛ.dEƛ is the energy, for wavelengths lying between ƛ and ƛ+dƛ emitted per second per unit surface area of black body.   

The wavelengths at different parts of the spectrum was calculated by "Prism Dispersion Formula".

The experiment results are as follows:

1) The emission from a Black body at any temperature is composed of Radiation from all wavelengths.

2) At a given temperature, the energy is not uniformly distributed. As the temperature of the black body increases, the intensity of radiation for each wavelength increases. This shows that the total amount of energy is radiated per unit area per unit time increases with rise of temperature.

i.e., T 𝛼 Eƛ

3) The total energy of radiation at any temperature is given by the area between the curve corresponding to that temperature and horizontal axis. The increase in area found in accordance with Stefans law.

4) The amount of radiant energy emitted is small at very short and very long wavelengths. At a particular temperature, the spectral radiance Eƛ is maximum at particular wavelength ƛm. Most of the energy is emitted at wavelengths not very  different from ƛm.

5) The wavelength corresponding to the maximum energy represented by the peak of the curve shifts towards shorter wavelengths as the temperature increases. This is called Wiens Displacement. According to this law ƛm x T = constant.

This shows that as the temperature is increased, the black body emits the radiation of shorter wavelengths such that the product of temperature 'T' and maximum wavelength ƛm is a constant. 

The constant is called Wiens Displacement constant and has value 0.2896 x 10⁻² mK. 

There is change in wavelength due to Doppler effect.

Laws Related to Black Body:

a) Kirchoffs Law
b) Stefan-Boltzmann law
c) Wiens Law
d) Rayleigh-Jeans Law
e) Plancks Law

Important points to be noted:

i)  Wiens formula agrees in short wavelength region.
ii)  Rayleigh-Jeans formula agrees for long wavelength region.
iii) Plancks formula covers the entire region.
iv) When radiation is passed through a black body is passed through a prism, acontinuous spectrum is obtained. The energy is distributed in various wavelengths varying from 0 to infinity.
v) The law that connects the intensity with the wavelength is known as law of distribution of intensity of black body radiation.
vi) According to Stefans law, u=𝝈T⁴ where 𝝈 is Stefans constant and 'u' is energy density.