In 1900 Max Plank found a theoretical formula that exactly describes the intensity of light of various frequencies emitted by a hot solid at different temperature.According to plan,the atoms of the solid oscillate or vibrate with a definite frquency, depending on the solid.But in order to reproduce the results of experiments on , he found it necessary to accept a strange idea.A vibrating atom could have only certain energies E, those allowed by the formula

E= nhv where h is a constant now called plank's constant value is 6.6 x 1o^-34 J

v= frequency

the value of n must be a whole number.

The number symbolized by n are called quantum numbers.The vibrational energies of the atoms are said to be quantized; that is possible energies are limited to certain values/

Albert einstein on the other hand, boldly extended Plank's work to include the structure of light itself.He postulated that light consists of quanta (now called Photons) or particles of electrmagnetic energy with energy E proportional to the observed frequency of light.

E= hv

or E = h c/ lambda where lambda is wavelenght & c= speed of light