I have a homework problem that goes like this. A beam of electrons is incident upon a gas of hydrogen atoms. What minimum speed must the electrons have to cause the emission of 656 nm light from the 3 -> 2 transition of hydrogen? So I had a couple of thoughts 1) It was the velocity needed to have state 3 orbit, but that's not right. 2) It was the velocity needed to have n=2 orbit, but that's not right either. 3) It was the velocity such that an electron had KE equal to the difference in E of states 3 and 2 but that's not it either. I suppose the wavelength of the light has something to do with this but it's just the wavelength of hydrogen emission 3->2 so I dunno what that tells you.