An electron jump from an outermost energy level to the innermost or ground level would emit the highest frequency photon. A greater the drop in energy levels results in a higher frequency of the emitted photon.
A hydrogen atom becomes ionized when its electrons have enough energy to jump out of the outermost energy level and become free of the atom. An electron near to escaping the atom has the most energy and therefore emits a high-energy photon if it drops all the way to level one. If the photon contains more energy, it has a higher frequency. Also, the lower energy levels are farther apart than the upper levels. This means that more energy is emitted in an electron fall from level two to level one than in a fall from level four to level three, and so on.