At the top of the applet you will see a graph of the potential, along with horizontal lines showing the unperturbed energy levels. By default it is an infinite square well (zero everywhere inside, infinite at the edges). Below that you will see an arrow showing the direction and magnitude of the electric field, and the current caused by the particle's motion. Below that you will see the probability distribution of the particle's position. At the bottom of the screen is a set of phasors showing the current particle state, and a set of circles showing possible final states.
When the applet starts up, there is an incoming wave which is at the right frequency (the Bohr frequency) to transition the particle from the ground state to the first excited state. Once it reaches the first excited state, it transitions back to the ground state, and so on.
To view another transition, double-click the appropriate state phasor to put the particle in that state. Then click the final state underneath it. This will start an incoming wave with the right frequency to make that transition.
Hydrogen Atom Applet.
Atomic Transitions Applet.
1-D Quantum Crystal Applet.
Other quantum mechanics demonstrations.