History of Spectra

Schematic of the transitions between the energy levels in the hydrogen atom. The transition from 2 = 2 to i = 1 emits a photon with a wavelength of 121.5 ran which is called the L)man-a for short because it is the principal line in the series. The transition from i2 = 3 to nj = 2 emits a photon with a wavelength of 656.3 nm and is usually referred to as the H-ct line instead of the Balmer-a line. 

Before the twentieth century, there was no theoretical explanation for these observed spectral lines or why other atoms should produce their own characteristic spectra. Finally, in 1913, Niels Bohr developed the first theoretical model that offered a basis for imder-standing these observations. 

---