Question

In a photon electron collision
(a) Only total energy is conserved.
(b) Only total momentum is conserved.
(c) Both total energy and total momentum are conserved.
(d) Both total momentum and total energy are not conserved.

Answer 1

Hint: To find information related to conservation of energy and momentum in a photon electron collision, we will take help of Compton Effect to get the correct option. The energy of a photon during collision is transferred to the electron and the same happens with momentum also, which results in conservation.

Complete answer:
Photon: Photon is a very basic particle actually and on this basis its mass is taken as zero. The important part of this is that it moves with a speed of light in a vacuum.

Electron: An electron is basically a part of an atom which always has a negative charge over it. The value of each electron is taken by us as $1.6\times {{10}^{-19}}C$. Its mass is lower than that of protons and neutrons.

Compton Effect: Photon electron collision is a Compton Effect. In this effect, what happens is the collision between a photon and an electron takes place.

In a path of a photon, the electron comes in its way resulting into the photon off from its path. This implies that the photon has now reduced its energy. For example we can say that a carbon atom has Compton Effect which is under photoelectric effect of 20keV the collision happens. After the collision it depends upon the energy sharing between a photon and an electron whether an electron gets forward in its path, or it gets deviated at some angle or it bounces back to its path in the opposite direction which is with respect to its forward move.
This explains the presence of photons in light particles. This describes momentum of photons. Under Compton Effect the photon transfers some of its energy into the electron. This energy brings elastic collision with a conserved state.

So, the correct answer is “Option C”.

Note:
During collision, the energy that is transferred from a photon to electron is called kinetic energy. And since the energy is being transferred so, it remains conserved in the system. As the momentum includes velocity and mass of any object, here the photon is that object and along with energy gives the same velocity to the electron which also conserves momentum in the system. It depends on the situation about the photon and electron’s change in positions. Either after colliding by electron the photon may get back to its path or they may travel in different directions.