What is the ratio of the time period of revolution of an electron in the second orbit of a hydrogen atom to that in the third orbit of a $He^{+}$ ion?

Newtonian mechanics is applicable to which objects and not applicable to which objects?

The surface of sodium metal is irradiated with radiation of wavelength $x$ nm. The kinetic energy of ejected electrons is $2.8 \times 10^{-20}$ $J$. The work function of sodium is $2.3$ eV. The value of $x$ is . . . . . . $\times 10^2$ nm.

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

The total energy of the electron of $H^{-}$ atom in the second quantum state is $-E_2$. The total energy of the $He^{+}$ atom in the third quantum state is

Emission transitions in the Paschen series end at orbit $n=3$ and start from orbit $n$ and can be represented as $\nu = 3.29 \times 10^{15} \, Hz \left(\frac{1}{3^2} - \frac{1}{n^2}\right)$. Calculate the value of $n$ if the transition is observed at $1285 \, nm$. Find the region of the spectrum.