What is the potential energy of the electron in the $L$ shell of the hydrogen atom? (in $eV$)

Energy of an electron is given by $E = -2.178 \times 10^{-18} \ J \left( \frac{Z^2}{n^2} \right)$. Wavelength of light required to excite an electron in a hydrogen atom from level $n = 1$ to $n = 2$ will be:

When atoms are bombarded with alpha particles,only a few in a million suffer deflection,while others pass through undeflected. This is because:

$A$ bulb emits electromagnetic radiation of $660 \, nm$ wavelength. The total energy of radiation is $3 \times 10^{-18} \, J$. The number of emitted photons will be :
$(h = 6.6 \times 10^{-34} \, J \cdot s, \, c = 3 \times 10^{8} \, m/s)$

The existence of a positively charged nucleus was established by:

The wavelength of radiation emitted when an electron falls from the $4^{th}$ Bohr orbit to the $2^{nd}$ in an $H$ atom is .............. $nm$.