This applet presents the user with an energy level diagram of an atom. The values of the levels (in eV) can be set by parameter tags. Relative transition probabilities can also be set by parameter tags. An active electron can be moved to any level by using the pointing device and then the electron will cascade down to the ground level according to the relative transition probabilities that have been set up. Each time the electron moves from a higher to lower energy level a photon will be emitted. The wavelength of the photon is indicated and some attempt is to color code the outgoing photon by the emitted wavelength.