New trends in physics teaching, v.4; The ... - unesdoc - Unesco

New Trends in Physics Teaching IV
This demonstration suggests to us that the information is encoded in the form of phase differences
and immediately establishes two important points. First (as in figure 3c) no information
can be encoded about dimensions much less than a wavelength of the radiation used and so it is
no use having a superb ‘optical’ system to form an image if the wavelength is too large. Second,
even if the radiation has been suitably chosen, it is still important to include all the waves from
the object. For example, if in figure 3b the aperture of the optical system cuts out all the waves
outside PQ, then the image wil be no better than the image formed in figure 3c with too long a
wavelength. Figure 4 shows some photographs taken to illustrate what can happen when either
the wavelength is too long or the aperture is too small. But to return to the patch of figure 2
again - if there are phase differences, why cannot we see interference fringes? Consider figure
3a; it is reminiscent of the arrangement for Young’s fringes and, under the right coherence
conditions, would produce sinusoidal interference fringes on the screen. If we now add all the
other possible pairs of points on the slide that make up the picture, each pair would produce a
sinusoidal fringe pattern on the screen. With the experiment as described so far - using an
ordinary projector - we do not see a pattern because the light is both spatially and temporally
incoherent. There is in fact a pattern on the screen at any instant - but it is a superposition of
many patterns because of the spatial extent of the source (lack of spatial coherence) and it is
changing at an enormous rate with time. If the light used were coherent both temporally and
spatially - as in our assumption for the theory - then the pattern of interference bands would be
stationary and visible, as indeed is shown (for the same slide) in figure 5.
Figure 5. Repeat of the experiment used to produce figure 2, but now the slide is illuminated by light that is temporally and
spatially coherent (laser source).
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