# Write two important properties of photons which are used to establish Einstein photoelectric equation.Use this equation to define threshold and stopping potential.

Dear Student,

According to Quantum theory, photons are elementary constituents of light. These having the following properties,
1) Photons are small packets of energy.
2) Photons collide elastically with material particles, Energy and Momentum are conserved on the average (Macroscopically).
So, The energy transfer happens instantaneously without any time lag.
Einstein's photoelectric effect is defined as,
When the light radiation incident on a metal, if the frequency of the light is above a certain value, electrons will be ejected from the material by absorbing the energy from the incident light.

According to Einstein, Kinetic energy of the electrons is having a range of energies instead of a single energy. Relation of the kinetic energy of emitted electron and incident photons is called Photoelectric equation. Which is as below,

Where,
h - Planck's constant
$\nu$ - frequency of the incident light
$\varphi$ is called Work function of the material. Which is defined as,
The amount of work has to done in order to remove the electron from the material.

This is constant for a particular material.

If we apply an Electric field in opposite direction, so that the electrons acted by some retardation. At a particular field the electron with maximum kinetic energy also stops before reaching anode, and as a result of this the current in the external circuit becomes zero. The potential difference needed to maintain in order to create this electric field is called "Stopping Potential".

According to Einstein, this stopping potential does not depends on the Intensity of the light radiation, it depends only on the frequency of the light incident.