Physics II Lab w answers_Part_64

Physics II Lab w answers_Part_64 - Appendix: Lasers...

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139 Appendix: Lasers Introduction: The light emitted from a conventional light source is(like sodium lamp) is said to be incoherent because the radiation emitted from different atoms do not, in general, bear any definite phase relationship with each other. On the other hand, the light emitted from a laser has a very high degree of coherence and is almost perfectly collimated. Laser is an acronym for Light Amplification by Stimulated Emission of Radiation. The basic principle involved in lasing action is the phenomenon of stimulated emission, which was predicted by Einestein in 1917. Einestein argued that when an atom is in the excited state, it can make a transition to a lower energy state through the emission of electromagnetic radiation; however, in contrast to the absorption process, the emission ca be occur in two different ways: i). The first is referred to as spontaneous emission in which an atom in the excited state emits radiation even in the absence of any incident. It is thus not stimulated by any incident signal but occurs spontaneously. ii) The second is referred to as stimulated emission in which an incident signal of appropriate frequency triggers an atom in an excited state to emit radiation. Using the phenomenon of stimulated emission, C.H. Townes and A.H. Schawlow, in 1958, worked out the principle of the laser. Stimulated Emission: Consider a gas enclosed in a vessel containing free atoms having a number of energy levels, at least one of which is metastable. By shining white light into this gas many can be excited, through resonance, from the ground state to excited states. As the electron drop back, many of them will become trapped in the metastable states. If the pumping light is
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This note was uploaded on 11/26/2011 for the course PHY 2053 taught by Professor Lind during the Fall '09 term at FSU.

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Physics II Lab w answers_Part_64 - Appendix: Lasers...

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