The colour of the positive column in a gas discharge tube depends on

The solar spectrum is an example of

When the potential energy of the second excited state of an atom is $108.8 \ eV$,the recoil momentum of the atom is ...... $kg \ m/s$.

$A$ beam of ultraviolet light of all wavelengths passes through hydrogen gas at room temperature,in the $x-$ direction. Assume that all photons emitted due to electron transition inside the gas emerge in the $y-$ direction. Let $A$ and $B$ denote the lights emerging from the gas in the $x$ and $y$ directions respectively.

Of the following pairs of species,which one will have the same electronic configuration for both members?

In a $He-Ne$ laser,the metastable state exists in: