The atomic hydrogen emits a line spectrum consisting of various series. Which series of hydrogen atomic spectra is lying in the visible region?

If an electron is moving in the n^th orbit of the hydrogen atom, then its velocity (v_n) for the n^th orbit is given as :

Which level of the single ionized carbon has the same energy as the ground state energy of hydrogen atom?

Calculate the time interval between 33% decay and 67% decay if half-life of a substance is 20 minutes.