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

The ratio of the radius of the first orbit in hydrogen to the radius of the first orbit in deuterium is .......

If the wave number of radiation emitted for the electron transition from an excited state $n_2$ to the first excited state $(n_1 = 2)$ of a hydrogen atom is $\frac{5x}{36} \ m^{-1}$, the wave number of radiation absorbed for the electron transition from the same excited state $n_2$ to the next immediate higher excited state $(n_3 = n_2 + 1)$ in $m^{-1}$ is:

When electromagnetic radiation of wavelength $300 \, nm$ falls on the surface of sodium,electrons are emitted with a kinetic energy of $1.68 \times 10^{5} \, J \, mol^{-1}$. What is the minimum energy needed to remove an electron from sodium? What is the maximum wavelength that will cause a photoelectron to be emitted?

If the series limit of wavelength of the Lyman series for the hydrogen atom is $912 \ \mathring{A}$,then the series limit of wavelength for the Balmer series of radiation which may be emitted is :

If an electron is moving in the $n^{th}$ orbit around the nucleus with a velocity $v$ and radius $r$,then the value of $n$ is .........