Optical spectra of atoms can be observed both in absorption and emission. When we transfer energy to the atoms of certain gases, they emit a characteristic spectrum consisting of lines. Further more, these atoms can absorb light having the same wavelength as the wavelength of the emitted ones. When an electric current is passed through a glass tube that contains hydrogen gas at low pressure, the tube gives off blue light. The emission spectrum of atomic hydrogen is divided into a number of spectral series, with wavelengths given by the Rydberg's formula. These observed spectral lines are due to electrons making a transition between two energy levels in the atom. The classification of the spectral series by using Rydberg's formula was important in the development of quantum mechanics as well as in astronomy for detecting the presence of hydrogen and calculating red shifts.
When the light from hydrogen discharge tube is passed through a spectrometer , five narrow bands of bright light are observed. Each of these narrow bands have characteristic wavelengths and colors. In this experiment, prism based spectrometer is used to measure the wavelengths of the emission lines of hydrogen. Before the wavelength measurement, the spectrometer is calibrated with mercury discharge tube and Hartmann's constants are calculated and verified. With the wavelength measurements of hydrogen lines, it is possible to compute the Rydberg's constant.