PARTICLE NATURE OF ELECTROMAGNETIC RADIATION: PHOTO ELECTRIC EFFECT

Particle Nature of Electromagnetic Radiation: 

Planck’s Quantum Theory

The important postulates of quantum theory proposed by Max Planck are:

1. Atoms and molecules could emit or absorb energy not in a continuous manner, but discontinuously 

in small packets of energy called quanta or photons. 

2. The energy (E ) of each quantum of radiation is proportional to its frequency (ν).

It is expressed by the equation, E = hν

Where ‘h’ is known as Planck’s constant and its value is 6.626×10–34 J s.

Photoelectric effect

It is the phenomenon of ejection of electrons by certain metals (like potassium, rubidium, caesium 

etc.) when light of suitable frequency incident on them. The electrons ejected are called photoelectrons. 

This phenomenon was first observed by H Hertz. The important characteristics of photoelectric effect are:

1. The electrons are ejected from the metal surface as soon as the beam of light strikes the surface. i.e., 

there is no time lag between the striking of light beam and the ejection of electrons from the metal 

surface.

2. The number of electrons ejected is proportional to the intensity or brightness of light.

3. For each metal, there is a minimum frequency known as threshold frequency *ν0], below which 

photoelectric effect is not observed.

4. The kinetic energy of the ejected electrons is directly proportional to the frequency of the incident 

light.