Question: what is the wavelength of a photon needed to excite an electron?
Answer: l = hc/E. A photon with an energy of 10.2 eV has a wavelength of 1.21 x 10-7 m, in the ultraviolet part of the spectrum. So when an electron wants to jump from n = 1 to n = 2, it must absorb a photon of ultraviolet light.