:In a certain design of helium-neon laser, the chamber containing these gases has a perfect mirror at one end. as usual, but only a window at the other, Beyond the window, is a region of free air space and then the second mirror, which is partially reflecting, allowing the beam to exit. The resonant cavity between the mirrors thus has a region free of the helium-neon gas-the "lasing material"-in which you can insert something. If you insert a sheet of clear plastic at any orientation in this region between the mirrors, the laser beam disappears. If the same sheet is placed in the beam outside the partially reflecting mirror, the beam passes through it, regardless of the orientation. Why?

The Quantum-mechanical model of an atom provides with a few quantum numbers which helps to determine the exact location of electron in an atom.

The first quantum number is the Principal Quantum number which is represented by$\text{n}$, it represents the shell number to which an electron belongs.

For a one-dimensional particle in a box, the n represents the shell to which electron belongs. The value of$n$ may be 1,2,3___n

In one electron atom like Hydrogen, Principal Quantum number holds a great significance. It can specify the radial distributions of the wave function and the energy values of the atom with different values It also specifies the radial dependence of the radial wave function.

Just like Principal quantum number$n$,there are three more quantum numbers, out of which two are- angular quantum number,$l$and the magnetic quantum number,$m_l$.

In the Hydrogen atom:

Angular quantum number, l: represents the shape of the orbital of Hydrogen

Magnetic quantum number, $m_l$: represents the energy levels in sub shell.