Question

What is population inversion and optical pumping? Explain.

Answer 1

This is a non-equilibrium condition of active medium used in laser in which number of atoms $N_2$ in the upper energy level momentarily exceeds the number of atoms $N_1$ in the lower energy level $N_1$ that is, $N_2>>N_1$.
Optical pumping: Optical pumping is the process of supplying optical energy(in general in the form of short slashes of light), to the active medium with a view to transfer into the state of population inversion.