Figure: The Bohr model

This applet shows the basic properties of the Bohr model. Electrons are "travelling" on circular orbits around the nucleus. Only special orbits are permitted. When an electron on a low orbit (corresponding to a low Energy) absorbs a photon, it goes to an orbit with higher energy. On the other hand, if an electron on a high orbit "jumps" to a lower orbit, it emits the energy difference in form of a photon that carries precisely the energy corresponding to the difference in energy levels.

Credit: Physics 2000, Colorado University at Boulder, http://www.colorado.edu/physics/2000/index.pl.

http://www.colorado.edu/physics/2000/quantumzone/bohr.html