Line+Spectra

=Line Spectra=

Theory
According to the Bohr model of the atom electrons exist around the nucleus at discrete energy levels. When these electrons absorb energy they move from lower energy “ground” states to less stable “excited” states. Inside a gas discharge tube, electrical energy is added to excite the atoms of the gas inside. Since the excited atoms are unstable the electrons drop back from the exited states to lower ground states. By energy conservation, energy must be released when the drop occurs. The energy is released as light. Since each ground or excited state exits at a very specific energy level then the differences between levels are also specific. Thus when energy is released it is released in specific amounts. The frequency of light is directly related to its energy by:

E = hf (where is E is energy, h is plank’s constant and f is frequency)

Shorter wavelengths of light, like blue or ultraviolet, carry more energy then longer wavelengths like yellow or red light. Since the energy released by an electron changing orbital’s is very specific the frequency of light produced is also very specific. These emissions, or line spectra, are unique to each atom.

Measurement
dsin(θ) = mλ (where m is maxima order and λ is wavelength) || ||
 * Line spectra are often measured using a diffraction grating. Diffraction utilizes the wave nature of light. Light passing through narrow slits (diffraction grating) will spread out behind the slits just as water waves entering a bay would. In most places the overlapping waves are out of phase and thus destructively interfere. However, because of the geometry, at specific locations (maxima) the path each light wave traveled is exactly one or more extra wavelengths. At these locations the waves constructively interfere creating a bright line spectra. Since different frequencies of light have different wavelengths, these line spectra maxima occur at different angles from the grating. The line spectra maxima repeat, with order 1 being the first maxima and order 2 the second. These angles can be measured and knowing d the slit distance, the wavelength of the line spectra can be determined using:
 * || Physicsits using spectroscopes (device with a mirror and a diffraction grating) to observe line spectra. ||

Applications
Line spectra can be used in astronomy to identify composition of unknown objects since each atom has a unique spectra. By red shift (Doppler effect) spectra that almost match known atom’s line spectra but are slightly moved to longer wavelengths the relative velocity between Earth and the unknown object can be determined.

Additional Resources:
Line spectra for each atom: [] High resolution line spectra for the sun: [] Discharge Lamp Simulation: []