Ionization potential of hydrogen atom is 13.6 V. Hydrogen atoms in the ground state are excited by monochromatic radiation of photon energy 12.1 eV. The spectral lines emitted by hydrogen atoms according to Bohr's theory will be

Three

Final energy of electron = −13.6 + 12.1 = −1.51 eV. which is corresponds to third level i.e. n = 3. Hence number of spectral lines emitted = $\frac{n(n-1)}{2}= \frac{3(3-1)}{2} = 3$