One of the most valuable experiments of modern physics is the analysis of absorption and emission of light by atoms.
- For example, an electric discharge can be created in a gas, so that the atoms begin to emit light with wavelengths characteristic of the gas.
- We see this effect in a neon sign, which is typically a glass tube filled with neon or a gas mixture, with electrodes for creating a discharge.
- If the intensity of the emitted light is measured as a function of wavelength, one finds a series of sharp lines rather than a continuous distribution of wavelengths.
- By the early 1900s the characteristic spectra for several atoms were well known.
- A portion of the measured emission spectrum for hydrogen is shown in Fig.given below, in which the vertical lines represent the positions of observed emission peaks on the wavelength scale.
- Photon energy hv is then related to wavelength by the dispersion relation λ = c/v.
- The lines in Fig. appear in several groups labeled the Lyman, Balmer, and Paschen series after their early investigators.
- Once the hydrogen spectrum was established, scientists noticed several interesting relationships among the lines.
The various series in the spectrum were observed to follow certain empirical forms:
where R is a constant called the Rydberg constant (R = 109,678 cm-1).
If the photon energies hv are plotted for successive values of the integer n, we notice that each energy can be obtained by taking sums and differences of other photon energies in the spectrum (Fig. given below).