What is Planck's Quantum Theory ?

Explanation:

Maxwell Planck gave a quantum theory of radiation(later on extended by Einstein) which explains phenomena like black body radiation, photoelectric effect, etc.

According to Planck’s Quantum theory:

Photon- A photon is an elementary particle that is a quantum of the electromagnetic field.
A hot body emits energy discontinuously in the form of small packets of energy called quanta.
Energy is directly proportional to the frequency.

E            or     E = h

Where, E= Energy
            h= Planck's constant= 6.62  10-34   Joule . sec
            = Frequency of radiation

Energy emitted / absorbed by a body can be one quantum ie E = h or whole multiple of h ie 2h, 3h, 4h ……nh

Also,                                                      E=h= hc= hc
 
Where, C= Speed of light= 3108
            = Wavelength
             = Wave no.

Final answer:

According to this theory different atoms and molecules can emit or absorb energy in discrete quantities only.