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