Unformatted text preview: d this phenomenon as being due to the fact that the radiation emitted by a single atom has a fixed phase angle,
period, , but only lasts a finite time , combined with the fact that there is generally no correlation between the phase angles of the radiation emitted by different atoms. Alternatively, we can account for the variation in the phase angle in terms of the finite
bandwidth of the light source. To be more exact, because the light emitted by the source consists of a
superposition of sinusoidal waves of frequencies extending over the range
to
, even
if all the component waves start off in phase, the phases will be completely scrambled after a time period has elapsed. In effect, what we are saying is that a practical monochromatic light source is
temporally coherent on timescales much less than its characteristic coherence t ime,
typically of order (which, for visible light, is seconds), and temporally incoherent on timescales much greater than . Incidentally, two waves are said to be coherent if their phase difference is constant in time, andincoherent if their phase
difference varies significantly in time. In this case, the two waves in question are the same wave observed at two
different times.
What effect does the temporal incoherence of a practical monochromatic light source on timescales greater than
seconds have on the two slit interference patterns discussed in the previous section? Consider the
case of oblique incidence. According to Equation (1008), the phase angles,
and , , of the cylindrical waves emitted by each slit are subject to abrupt random changes on timescales much greater than , because the phase angle, , of the plane wave that illuminates the two slits is subject to identical changes. Nevertheless, the relative phas...
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This note was uploaded on 08/25/2013 for the course PHY 315 taught by Professor Staff during the Fall '08 term at University of Texas.
 Fall '08
 Staff
 Photon, Waves And Optics, Light

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