The ratio of the velocity of the electron in the first Bohr orbit to that in the second Bohr orbit of hydrogen atom is

The shortest wavelength in the Balmer series of hydrogen atom is equal to the shortest wavelength in the Brackett series of a hydrogen like atom of atomic number $$\mathrm{z}$$. The value of $$\mathrm{z}$$ is

The ratio of longest to shortest wavelength emitted in Paschen series of hydrogen atom is

The force acting on the electron in hydrogen atom (Bohr' theory) is related to the principle quantum number '$$n$$' as