What is the wave number of the lowest energy transition in the Balmer series?

If the wavelength of an electromagnetic radiation is $2000 \ \text{Å}$, what is its energy in ergs?

The energy of an electron in the ground state $(n=1)$ for $He^{+}$ ion is $-x \ J$. What is the energy for an electron in the $n=2$ state for $Be^{3+}$ ion in $J$?

If the threshold wavelength for a metal is $\lambda_0$ and the wavelength of incident light is $\lambda$,what will be the maximum velocity of the emitted electrons?

The radius of $3^{rd}$ orbit of hydrogen atom is $R \text{ pm}$. The radius of $2^{nd}$ orbit of $He^{+}$ ion (in $\text{pm}$) is

If the radius of the electron orbit in the excited state of a hydrogen atom is $476.1 \ pm$, the energy of the electron in that excited state in $J$ is (Radius and energy of the electron in the first orbit of a hydrogen atom are $52.9 \ pm$ and $-2.18 \times 10^{-18} \ J$ respectively).