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

The ionisation energy of an electron in the ground state of a helium atom is $24.6 eV$. The energy required to remove both electrons is: (in $eV$)

The graph between the square root of the frequency of a specific line of characteristic spectrum of $X$-rays and the atomic number of the target will be

Line spectrum contains information about

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

Match the appropriate pairs from List-$I$ and List-$II$:

List-$I$	List-$II$
$1$. Nitrogen molecules	$(A)$ Continuous spectrum
$2$. Incandescent solids	$(B)$ Absorption spectrum
$3$. Fraunhofer lines	$(C)$ Band spectrum
$4$. Electric arc between iron rods	$(D)$ Emission spectrum