What are planck's postulates..??

Classical wave theory of radiations was not able to explain black body radiations photo-electric effect, etc. In order to explain these facts Planck (1901) gave a theory called Planck’s quantum theory of radiation.

Max Planck in 1901 studied the energy of radiations of different frequencies radiated from hot bodies, known as black bodies. On the basis of these studies, Planck postulated a theory known as Planck’s quantum theory of radiations.


The various postulates of quantum theory of radiations are as follows:

(i)  A radiation has energy. As light and heat are radiations, they are also associated with energy.

(ii)  Radiant energy is not emitted or absorbed continuously but discontinuously in the form of small packets called photons. Photon is not a material body but is considered to be a mass less packet of energy.

(iii)  The energy E of a photon is related to the frequency of radiation, v; the two being related as E = h v, Where h is planck’s constant.

(iv) Whenever a body emits or absorbs energy, it does so in whole number multiples of photons, i.e., nhn where n = 1, 2, 3 and never 1.2, 2.5, 3.7 etc.

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Planck   postulated that the energy of oscillators in a blackbody is quantized by


where n = 1, 2, 3, ..., h is Planck's constant, and   is the frequency, and used this postulate in his derivation of the Planck law of blackbody radiation. In fact, electromagnetic radiation is itself quantized, coming in packets known as photons and having energy


In the other hand, the energy of state n of quantum mechanical simple harmonic oscillator is actually given by the slightly modified form


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 The Planck postulate (or Planck's postulate), one of the fundamental principles of quantum mechanics, is the postulate that the energy of oscillators in a black body is quantized, and is given by


where   is an integer (1, 2, 3, ...),   is Planck's constant, and   (the Greek letter nu, not the Latin letter v) is the frequency of the oscillator.

The Planck Postulate was introduced by Max Planck in his derivation of his law of black body radiation in 1900. This assumption allowed Planck to derive a formula for the entire spectrum of the radiation emitted by a black body. Planck was unable to justify this assumption based on classical physics; he considered quantization as being purely a mathematical trick, rather than (as we now know) a fundamental change in our understanding of the world.[1]

In 1905 in one of his three most important papers, Albert Einstein adapted the Planck postulate to explain the photoelectric effect, but Einstein proposed that the energy of photons themselves was quantized, and that quantization was not merely a feature of microscopic oscillators. Planck's postulate was further applied to understanding the Compton effect, and was applied by Niels Bohr to explain the emission spectrum of the hydrogenatom and derive the correct value of the Rydberg constant.


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