BLACK BODY RADIATION

In 1897, Lummer and Pringsheim measured the intensities of different wavelengths of radiations emitted by black body using Bolometer (a device made from thermocouple) and plotted them.

He proposed that the energy density at wavelength l at time t, a being the proportionality constant. This equation is found to be valid for small wavelengths but fails for the longer wavelengths. Thus, all above attempts based on thermodynamics and electromagnetic theories failed to explain the energy distribution curves of black body radiation. Finally, in 1900 A.D., Max Planck explained these experimental results.

Planck’s hypothesis: Planck suggested that the walls of cavity emitting radiations are made of electric dipoles. According to there temperatures, different dipoles oscillate with different frequencies and emit

radiations of frequencies same as the frequencies of their oscillations. According to classical physics, an oscillator may possess any amount of energy in a continuously varying energy. But Planck hypothesized that the oscillator can possess only such discrete values of energies depending on its frequency. Thus, the energy of radiation of frequency f may be considered to be made up of many quanta of energy hf and a radiation may be made of quanta of different frequencies. The quantum of radiation energy is called ‘photon’. The radiation of different frequencies is the collection of photons of various frequencies. If an oscillator possesses energy 3hf, it may be considered equivalent to a collection of 3 photons.

Progress of Quantum Theory: - (i) In 1905, Einstein explained the photoelectric theory using Planck’s Quantum Theory. (ii) After the discovery of an electron, radioactivity in which a - and b - particles and g - rays are emitted was discovered. From the knowledge that the atom is neutral, it was concluded that the positive and negative charges in an atom must be in equal amount. (iii) Thomson and then Rutherford gave the model of an atom. Later, In 1913, Bohr introduced quantum theory to explain the structure of the atom. (iv) Frank and Hertz provided experimental support to Bohr’s theory. (v) In 1916, Einstein proposed that the photon also possesses quantized momentum. (vi) In 1923, Compton gave an experimental support to the photon theory by attributing the properties of a particle to a photon. (vii) In 1923, De-Broglie hypothesized wave nature of material particles. (viii) In 1927, Davisson and Germer gave experimental proof of De Broglie’s matter waves. (ix) Heisenberg gave uncertainty principle and Schrodinger established a differential equation to understand the behaviour of matter particles and within six months solved some problems regarding them. (x) Max Born and Dirac also contributed in the development of quantum mechanics. (xi) The relativistic quantum mechanics was also developed. Einstein’s theory of relativity also played a useful role in the development of the modern physics.

Emission of Electrons: -

Metals have free electrons (valence electrons). To be freed from the metal, they need some minimum energy, called work function of the metal. Its unit is electron volt (eV).

The work function of a metal depends upon the type of the metal and its temperature. To free them, energy may be supplied by any one of the following methods.

(1) Thermionic Emission:

When current is passed through a filament so that it gets heated sufficiently, free electrons from the metal of the filament get emitted. Such emission occurs in diode, triode and TV tube (Cathode ray tube).

(2) Field Emission:

When a metal is subjected to strong electric fields of the order of 108 V / m, electrons get emitted from the metal.

(3) Photoelectric Emission:

When electromagnetic radiation of sufficiently high frequency is incident on a clean metal surface, free electrons are emitted from the surface. This method is called photoelectric emission and the electrons so emitted are called the photoelectrons.