Support the Monkey! Tell All your Friends and Teachers

Help / FAQ



The development in Physics from 17th to 19th century reached its pinnacle towards the end of the 19th century. The Newtonian paradigm of synthesizing different branches such as Mechanics, Heat, Sound, Electricity, Magnetism and Optics appeared to be at its zenith. However, three observations were defying this paradigm, the black body radiation spectrum, Balmer's formula for line Spectrum of Hydrogen Atom and Michelson- Morley's null results were challenges to the very foundations of Newtonian concepts of Space, Time and Mass on one hand and Newton's motion of determinism on the other.

The resolution to these "three dark clouds" according to Lord Kelvin, unfolds a new era in Physics since the beginning of the 20th Century, the generic term for subsequent developments is Modern Physics, and in contrast, Physics, upto the 19th Century is called Classical Physics.

The Spectrum of radiation from a black body is found to be as shown in Figure 1.

u : Energy density of radiation.

: Frequency of radiation.

The overall features of these curves were explained satisfactorily by Thermodynamical arguments, viz. ()max T called Wien's displacement law and total radiated power proportional to T i.e. Stefan- Boltzman Law; however Rayleigh- Jeans Law derived on the basis of the classical Electro- magnetic theory is unable to account for the radiation spectrum since

(where K: Boltzman's Constant)

Max Planck made a radical hypothesis that the exchange of energy between oscillation in the walls of the black body and radiation occurs in a discrete manner unlike the classical assumption of continuous exchange.

Planck hypothesized that the oscillations can have energies h, 2h, 3h,.... where 'h' is constant known as Planck's Constant. Therefore radiation can absorb or transfer energy also in such quantas or bundles. Planck derived the formula

Equation (2) is known as Planck's radiation law. It is in excellent agreement with the observed Spectrum.

The introduction of Planck's hypothesis is the beginning of Quantum Theory which is one of the pillars of Modern Physics.


[next page]


Chapter 29 :Planck's Quantum Theory of Radiation

Chapter 30

All Contents Copyright © All rights reserved.
Further Distribution Is Strictly Prohibited.

In Association with