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

New Trends in Physics Teaching IV
much more profitable and already transmission in optical fibres is being classified as ‘single mode’
‘multi mode’, etc. just as for radio-frequency waveguides.
Finally, in this brief survey in which we are really concentrating on the optical aspects rather
than the communication aspects, we should remind ourselves that dispersion occurs in media
such as glass. Consequently, unless the light used is of a very narrow defined frequency, signal
pulses may become diffuse because the different frequencies present propagate at different
velocities. This is one of the reasons why lasers, producing a very narrow frequency band, are an
important component in optical communication systems. The high intensity is, of course, an
added bonus.
Imaging systems
Perhaps the most familiar imaging system for most people is the television set, and the advent of
television games and domestic video recorders has brought pieces of highly sophisticated technology
into people’s homes. Colour television itself is a remarkable feat; the problems of transmitting
three separate coloured moving images in precise register with each other is a formidable one.
To record such images on tape or disc and play them back with equipment cheap enough to be
brought on a domestic scale is a triumph of technology. And the facility for slow motion or still
picture replay means that a form of image processing is now available relatively cheaply.
But in order to illustrate the exciting potential of imaging systems in a brief section, I have
chosen to concentrate on only three topics; electron microscopy, medical imaging and pictures
from space.
In its optical essentials, an electron microscope obeys the normal criteria for an image-forming
system. The radiation is electrons and, if we assume that an accelerating voltage of say 120 kV
is used, the equivalent wavelength of the electrons will be about 3 picometres; that is, of the
order of 1/100 of the spacing between atoms in solid matter. In principle, therefore, the electron
microscope should be able to image atomic locations quite clearly; all the information could be
satisfactorily coded. However, you wil recall that we pointed out (p. 000) a second limitation on
resolution, namely that all the information encoded in the radiation might not enter the
recombination section of the system. This is exactly what happens with electron microscopes;
the electron lenses can only be made with very tiny apertures and, in practice, really complete
resolution of atoms in all forms of material remains out of reach. In certain very special cases, it
can be done.
The other major problem in electron microscopy was that of achieving a large depth of field
when focusing. The scanning electron microscopes - in which the electron optical system is used
to produce a very fine beam of electrons which scan the object and, after scattering, are collected
by a counting device from which the signals are presented on atelevision type display - deal
with this problem and, incidentally, produce some of the most attractive and exciting pictures
in the process.
In electron microscopes, just as with optical microscopes, it is important to remember the
Abbe principle. That is that the final image is not necessarily an image of the object. It is in fact
an image of such an object as would give a scattering or diffraction pattern corresponding to that
portion of the pattern of the real object that enters the system. An obvious example would be an
attempt to image a diffraction grating whose transmission function was a square wave. Its scattering
pattern would be a series of regularly spaced orders of diffraction; but if only one order on
each side of the centre entered the recombination system, the image could only be a sinusoidal
distribution of the same period as the square wave. False images are all too easy to obtain if care
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