When an electron transitions from the $n^{th}$ orbit to the $2^{nd}$ orbit in a $Li^{2+}$ ion,$15$ spectral lines are obtained. What is the value of $n$?

The energy of the second Bohr orbit of the hydrogen atom is $-328 \ kJ \ mol^{-1}$. Hence,the energy of the fourth Bohr orbit would be .............. $kJ \ mol^{-1}$.

What will be the number of photons required to obtain $1 \ J$ of energy by radiation having a wavelength of $4000 \ \mathring{A}$?

The number of photons emitted by a monochromatic (single frequency) infrared range finder of power $1 \, mW$ and wavelength of $1000 \, nm$,in $0.1 \, s$ is $x \times 10^{13}$. The value of $x$ is ..... .
(Nearest integer)
$(h=6.63 \times 10^{-34} \, J \, s, c=3.00 \times 10^{8} \, m \, s^{-1})$

The dual nature of photons is described by:

What is the shortest wavelength in the $Pfund$ series of $He^{+}$ ion?