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

Optics regained
Consider any two points A and B in figure 3a that are rather far apart on the slide used in
figure 2. At point P on the screen, the waves from A and B wil be in phase since they have
travelled the same distance. At point Q however there wil be a phase difference corresponding to
an extra path distance d. Let us suppose that this is just one half wavelength X/2. In figure 3b
the points A' and B' are much closer together than A and B. Again the waves are in phase at P
but at Q the path difference d' is very much less than d in figure 3a. In fact we have to move out
to Q' before the path difference is equal to h/2. In figure 3c the points A'' and B" are considerable
closer together than one wavelength of the radiation used and so, even if we were able to
collect all possible waves including those going off in the direction of Q", the path difference
would be less than one half wavelength.
Figure 4a. A regular object. b. The scattering path of a in coherent light. c. A restricted portion of b. d. The image formed on
recombining c. e. Further reduction of b. f: The image formed on recombining e: note particularly that the centre group now
appears to be 3 x 3 rather than the 5 x 5 of the real object. [22.]
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