**CBSE NOTES CLASS 12 PHYSICS **

**CHAPTER 11 DUAL NATURE OF RADIATION AND MATTER**

**Einstein’s photoelectric equation: energy quantum of radiation**

- Photoelectric emission does not take place by continuous absorption of energy from radiation. Radiation energy is made up of discrete units called
**quanta of energy of radiation**. - Each quantum of radiant energy has energy hν, where h is Planck’s constant and ν the frequency of light. In photoelectric effect, an electron absorbs a quantum of energy (hν of radiation. If this quantum of energy absorbed exceeds the minimum energy needed for the electron to escape from the metal surface (work function W
_{0}), the electron is emitted with maximum kinetic energyK

_{max}= hν – W_{o} - Intensity of radiation is proportional to the number of energy quanta per unit area per unit time. The greater the number of energy quanta available, the greater is the number of electrons absorbing the energy quanta and greater is the number of electrons coming out of the metal (for ν > ν
_{o}). This explains why, for ν > ν_{o}, photoelectric current is proportional to intensity. - Whatever may be the intensity i.e., the number of quanta of radiation per unit area per unit time, photoelectric emission is instantaneous. Low intensity does not mean delay in emission, since the basic elementary process is the same. Intensity only determines how many electrons are able to participate in the elementary process (absorption of a light quantum by a single electron) and, therefore, the photoelectric current.

**This theory accounts for all the observations of the photoelectric effect.**