For hydrogen, an electron jump from the second energy level down to the first emits a photon within the ultraviolet range of the spectrum at a wavelength of 1.21 x 10-7 meters, or 121 nanometers. The emitted photon has exactly 10.2 electron volts of energy.
Electrons residing at the first energy level have -13.6 electron volts of energy, and those residing at the second energy level have -3.4 electron volts of energy. The energy difference from level one to level two is equal to the amount of energy required for an electron to jump between the levels. A photon emits light at a wavelength directly corresponding to its energy value. The wavelength in meters is determined by multiplying the speed of light and Planck's constant, then dividing by the photon's energy in electron volts.
Interestingly, for an electron to jump back up, from level one to level two, requires a photon of the same energy that was emitted in the jump down. However, an upward jump requires the absorption of an ultraviolet photon, rather than emission of one.