The Bohr model of the hydrogen atom is very simplistic. Despite its limitations, applying basic principles of physics to the model results in reasonably accurate predictions for the electron energy levels, for in the Bohr model electrons can exist only in certain well-defined energies. The energy levels it predicts are shown at the right, and the circular orbits of the Bohr model are at the left.
The electron can make a transition to another energy level by absorbing or emitting a photon. The energy carried by the photon corresponds to the change in energy of the electron. As a result, the energy (and hence the wavelength) of light emitted and absorbed by an atom can take on only specific values corresponding to the differences between atomic energy levels.
Use the absorb and emit buttons to move your model between energy levels, and note the corresponding transitions on the energy level diagram. The model does not show the n=1 level because transitions to that level do not emit visible light.
Use the absorb and emit buttons to move your model to the n=3 level. Is it possible for this atom to release 0.7 eV (electron volts) of energy? How about to release 1.9 eV of energy? You can see that the atom cannot change energy by any amount, but only by very specific amounts.