Planck's Quantum theory

Introduction 

Modern physics is a branch of physics which deals with the discoveries in twentieth century. One of the best discoveries in twentieth century is "Photoelectric effect" explained by Albert Einstein. He received Nobel Prize for photoelectric effect in 1921.

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Black-body Radiation - 

When a white ray falls on a normal body; the three cases occurs-

1. Absorption- some part of the white light is absorbed by the normal body.
2. Reflection- only one color is reflected by the body because the body already have that color. Reflected color enters into our eyes and we are able to perceive the actual color of the body.
3. Transmission- some part of the white light goes through the normal body. 

              But when a white ray falls on a body which absorbs all the white ray is known as black body. E.g. Charcoal, layer made by carbon etc.

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Using the above concept we can conclude that when we give heat to a body it radiates all possible wavelength as shown in left fig above. The intensity of a particular wavelength is very high because of which we perceive the body's actual color as shown in right fig above.

E.g. (a) When we heat an iron rod; it appears red this is because at this temperature the mostly radiated wavelength is equal to red. When we increase heat; iron rod start appears to be blue. Again  this is because at this temperature the mostly radiated wavelength is equal to blue.This method is also used for finding the temperature of Stars.

        Now some scientist like Rayleigh & Jeans makes theoretical graph as shown below assuming light as a wave. The graph is similar for high wavelength but different for low wavelength. Then Planck introduce his quantum theory using which graph exactly match with practical one.

                                       

Planck's quantum theory -

1. Radiation from a body; which is emitted; not continuously but discontinuously in the form of small energy packets called quantum (quanta- plural of quantum).
2. The energy of quantum is directly proportional to the frequency of radiation.

                                         E ∝ 𝜈   or    E = h𝜈
                              where h is Planck's constant 
                              h = 6.626 ✖ 10⁻³⁴ Js
      3. A body can absorb or emit quantum in whole number only.

          i.e; 1 h𝜈, 2 h𝜈, 3 h𝜈...........so on

          not 1.5 h𝜈, 2.5 h𝜈, 3.5 h𝜈..............so on