The whole of the preceding discussion is premised on

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Unformatted text preview: temporally and spatially incoherent on timescales much longer than a typical atomic coherence time, which is about seconds. This is generally the case. However, there is one type of light source- - namely, a laser- - for which this is not necessarily the case. In a laser (in single- mode operation), excited atoms are stimulated in such a manner that they emit radiation that is both temporally and spatially coherent on timescales much longer than the relevant atomic coherence time. Let us consider the two- slit far- field interference pattern generated by an extended coherent light source of angular extent . In this case, as is readily demonstrated (see Exercise 2), Equation (1017) is replaced by (1022) It follows, from Equation (1018), that the visibility of the interference pattern is unity: that is, the pattern is sharply defined, irrespective of the angular extent of the light source. (However, the overall brightness of the pattern is considerably reduced when .) It follows that lasers generally produce much clearer interference patterns than conventional incoherent light sources....
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