An electron of mass m_e has a speed u_n in the n^ th Bohr orbit of a hydrogen atom. The normalized speed V_n is given as V_n=u_n / c=1 / 685 and the mass of the electron in the units of energy is given as M_e=m_e c^2=0.51 10^6 eV , where c is the velocity of light in vacuum. It is given that ^2 2 ₀ h c =1 / 137 . The Planck's constant h is given as 4.13 10⁻¹⁵ eV -sec. The electron makes a transition from n^ th orbit