The range of the nuclear force is

Considering the Bohr's model of hydrogen atom, the ratio of velocities of electrons orbiting in the 4th orbit to that in the 9 th orbit is

What is the mass number of the nucleus having radius equal to $\frac{1}{3}$ of that of ${ }^{189} \mathrm{Os}$ ?

The difference in the wavelength between the maximum and minimum of Balmer series (use $R_H=1 \times 10^7 \mathrm{~m}^{-1}$ )