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

New Trends in Physics Teaching IV
It can be shown that something very like this pair of operations occurs in all image focusing
systems, whatever the radiation used, be it radar to see through fog, infra-red radiation to see in
the dark, ultrasonic scanning to see inside the human body, or X-ray crystallography to look at
atoms. In a moment, we wil pick out just one or two examples to illustrate this theme. But,
first, let us take a very elementary look at the theoretical aspect. We need to ask the question
‘If the information is all there in the patch of figure 2, in what form does it exist?’. We shall
examine first what would happen if the slide were illuminated by parallel, monochromatic waves.
Figure 3. Path differences for waves scattered from two points A and B for various separations between A and B. In a, the points
are five wavelengths apart and in moving from P to Q on the screen, the path difference becomes half a wavelength. In b, the
points are two wavelengths apart and it is necessary to move from P to Q’ in order to make the path difference half a wavelength.
In c, the points are half a wavelength apart and even at 90’ to the original direction the maximum path difference is clearly half a
wavelength.
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