Question

Assertion: In the He-Ne laser, population inversion takes place between the energy levels of neon atoms.
Reason: Helium atoms have a meta-stable energy level.
A) If both assertion and reason are correct and reason is the correct explanation of assertion.
B) If both assertion and reason are correct but reason is not the correct explanation of assertion.
C) If the assertion is correct but reason is incorrect.
D) If the assertion is incorrect but reason is correct.

Answer 1

Hint: In a laser, electron transitions take place from the same excited state to the ground state. This excited state is known as the meta-stable energy level.

Complete answer:
“Laser” is an acronym for light amplification by stimulated emission of radiation. A laser is an intense coherent beam of light that is created when the electrons in atoms in special glasses, crystals, or gases absorb energy from an external source. This excites the electrons in the atoms to rise to an excited state.
To accumulate enough electrons to be excited to the same level, we need a meta-stable state. The meta-stable state traps and holds the excitation energy, building up a population inversion that can be further stimulated to emit radiation, when the electrons drop back to the ground state.
In a He-Ne laser, external power pumps the electrons up to a higher excited meta-stable state which has a lifetime of ${10^{ - 8}}$ seconds. The energy is provided by the kinetic energy of the Helium molecules' kinetic energy to the Neon atoms which excite the electrons in the ground state of the Neon atom up to the excited meta-stable state. This leads to a population inversion as now there will be more electrons in an excited state than in the ground state.
When a lot of electrons accumulate at this energy level, the transition of one electron back to the ground state causes all the other electrons to transition too, and hence we will get a laser light.
Hence the assertion and reason are correct but the reason is not the correct explanation of assertion since it is the Neon atoms which have a meta-stable state and not Helium atoms.

 So option (B) is the correct choice.

 Note: Laser light is much different from the normal white light that we see. Laser light is much more focused, coherent, and can travel for long distances without getting scattered. Meta-stable states are the only possible method of creating a population inversion that is required for laser action.