Bohr Model of the Atom

White light has a continuous spectrum of colours.

Fraunhofer observed dark lines in the light spectrum of the sun.

The emission and absorption spectra of atoms such as Hydrogen differ from the continuous spectrum of white light.

When gases such as hydrogen, helium and neon were heated, or when metals ignited in a flame, emission spectra were observed. Instead of a continuous spectrum, a relatively small number of coloured lines were observed.

After Fraunhofer, astronomers observed absorption spectra in all types of stars.

Neils Bohr realised that emission and absorption spectra were an indicator for how the electrons in an atom behaved. The observed spectra gave a clue to the structure of the atom.
(1) absorption spectra showed that atoms such as the hydrogen atom were only capable of absorbing a small number of different frequencies, and therefore energies.
(2) the emission spectra of hydrogen showed that hydrogen was also capable of emitting photons of the exact energy value that it was able to absorb.

Bohr proposed that:
(1) a number of allowable electron orbits of different radii exist for each atom labeled n=1,2,3,... And the electron may occupy only these orbits.
(2) An electron ordinarily occupies the lowest energy orbit available. In this orbit of lowest energy, it does not radiate energy.
(3) Electromagnetic radiation can be absorbed by an atom when its photon energy is exactly equal to the difference in energies between an occupied orbit and a higher energy orbit. This photon absorption results in an excited atom.
(4) Electromagnetic radiation is emitted by an excited atom when an electron falls from a higher energy level to a lower energy level. The photon energy will be exactly equal to the energy difference between the electron's initial and final level.