Laser light containing photons of energy 1.96 eV (red) is incident on a low-temp

Question

Laser light containing photons of energy 1.96 eV (red) is incident on a low-temperature gas of atomic hydrogen. Because of its low temperature, nearly all of the atoms have energies equal to the ground state energy. The photons will (A) be absorbed by hydrogen atoms in the gas. (B) will be partially absorbed by the gas, meaning that some photons will be absorbed but not all of them. (C) will pass through the gas without losing any energy or being absorbed. (D) will pass through the gas, but will lose kinetic energy as a result of colliding with electrons in the hydrogen atoms.

Explanation / Answer

Answer :(D)

At low temperature electron will be in the ground state whose binding energy is 13.6 eV.

To make a transition from n=1 to n=2 orbital electron needs= (13.6 ev)(1-1/22) =13.6x(3/4) ev =10.2 eV

To make a transition from n=1 to n=3 orbital electron needs= (13.6 ev)(1-1/32) =13.6x(8/9) ev =12.08 eV

For all heigher level transition energy will be heigher.

So if a laser light (1.96 ev) falls on low temperature gas, it have not enough energy to make a transition in Hydrogen energy levels. So laser will not absorbed by gas.

But during passing laser light lose enegy due to colliding with electron in hydrogen atom. This phenomena known as light matter interaction or Compton scattering.

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