|CHAPTER 29 : PLANCK'S QUANTUM THEORY OF RADIATION
INTRODUCTION TO MODERN PHYSICS
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
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.