If the electron in a hydrogen atom jumps from an orbit with level $n_1=2$ to an orbit with level $n_2=1$, the emitted radiation has a wavelength given by

$A$ $\mu$-meson of charge '$e$',mass $208 m_e$ moves in a circular orbit around a heavy nucleus having charge $+3e$. The quantum state '$n$' for which the radius of the orbit is the same as that of the first Bohr orbit for a hydrogen atom is [approximately]:

If the energy of an electron in the ground state is $-13.6 \text{ eV}$,find the speed of the electron in the fourth orbit of an $H$-atom.

In Bohr's model of the hydrogen atom,which of the following pairs of quantities are quantized?

In hydrogen atoms, the radius of the smallest orbit of the electron is $a_0$. The radius of the third orbit is:

The frequency of the $1^{st}$ line of the Balmer series in the ${H_2}$ atom is ${\nu _0}$. What is the frequency of the corresponding line emitted by a singly ionized ${He^+}$ atom?