By combining classical physics and some theories of his own Bohr was able introduce a revolutionary model for all atoms. Previous atomic models had the negatively charged electrons orbiting the positively charged nucleus. The man who invented this model of the atom, Ernst Rutherford, knew that this was true, but couldn't think of a better way to describe an atom. Using the spectrum of hydrogen, Bohr found that when a prism diffracted the light color was displayed at only four discrete increments. He saw that these results would fit his model if he assumed that the angular momentum of the electron could only occur in certain increments.
Angular momentum equals the product of mass, velocity, and orbital radius.
Using this he assigned the hydrogen atom energy levels consistent with the hydrogen emission spectrum. This gave way to Bohr historic equation for the energy levels available to the electron in the hydrogen atom. The n is an integer. The larger the value of n , the larger is the orbital radius. Z is the nuclear charge.