Question

Define the photoelectric effect and describe how Planck and Einstein contributed

Answer 1

Photoelectric effect(photon=light)
It is the phenomenon of ejection/release of electrons from certain metal surfaces (like potassium, rubidium, caesium etc.) when light of suitable frequency incident on them. The ejected electrons are called photoelectrons. . The important characteristics of photoelectric effect are:
1. There is no time lag between the striking/hitting of light beam and the ejection of electrons from the metal surface( according to classical physics, before Planck’s quantum theory it was like - if a dim light strike it will hit the metal surface and activate the electrons and after some time electrons will eject).
2. The number of electrons ejected is proportional to the intensity or brightness of light.
3. For every metal, there is a minimum frequency (known as threshold frequency [0]) below which photoelectric effect is not observed. ie a minimum energy required to remove an electron from the metal surface- threshold frequency. (but according to classical physics, was like if more number of light radiation comes, they would be able to eject the electron)
4. The kinetic energy of the ejected electrons are directly proportional to the frequency of the incident light.

Now, according to Planck’s quantum theory, energy transmitted in the form of small packets called quanta, that energy equal to h

E=h

h= Planck constant = 6.626x10^-34Js

frequency of radiation.

Based on Planck’s quantum theory, Einstein further explained Photo electric effect as follows

kinetic energy of the ejected electron is given by
K.E = h - h₀
Or, h = h₀ + ½ m_ev²
Where me is the mass of the electron and is the velocity of the ejected electron. when a beam of light incident on a metal surface it transfer the energy into electron as hand a part of it as kinetic energy to move the electron from the metal surface. Greater the energy possessed by the photon, greater will be transfer of energy to the electron and greater the kinetic energy of the ejected electron. That is always a radiation with a minimum energy required to eject an electron (work function), whatever will be their number.
A more intense beam of light contains larger number of photons, so the number of electrons ejected is also larger.

Simply, Einstein explained photo electric effect based on the planks quantum theory, E=h. A minimum energy required to eject an electron from the metal surface when a radiation of suitable frequency hit it.