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

Optics regained
human beings since we all use it. But it is also of special importance to scientists because one of
their aims is to by-pass the more subjective elements and arrive at objective conclusions; this
objectivity can only be achieved if we understand the nature of the subjective complications.
This approach to optics thus fits in well with the notion that physics courses should be designed
to be equally appealing to generalists and to budding scientists.
Optics offers a fine medium for bringing together a large number of apparently different
topics and of demonstrating their interconnections. A personal illustration wil perhaps make
this point quite forcibly. During the second half of the Second World War I was working for the
British Admiralty on the problem of radar jamming. This was the period when radar systems were
moving to higher and higher frequencies and devices such as wave-guides and horns began to
replace coaxial cables and arrays of di-poles as radiators. The problems were new, but I was able
to come to terms with quite a few of them by using my knowledge of optics; wave guides with
radiating slits were to me merely diffraction gratings; circularly polarized radiators were half
wave plates and nicol prisms! Thus my optical knowledge helped me to talk quite sensibly with
radio engineers and to make significant contributions. Later in life, I began to work on crystal
structure determination using X-ray diffraction and, here again, optics came to my aid; the
recognition that the apparenly formidable computations of the crystallographer are merely doing
what a lens does for visible light gives powerful insights into the potential and limitations of the
subject. Finally, my experiences in experimental optics led to an appreciation of the mathematical
processes underlying a great deal of modern optics - namely Fourier Transformation - and this
in turn stood me in good stead for understanding problems in Musical Acoustics which involve
essentially the same mathematical processes.
There are many other arguments that could be brought to bear but I think, on balance, it is
best to let the subsequent sections on the various possible topics speak for themselves. Let us
start by looking again at some of the key ideas that have provided the basis for modern optics.
THE SEEDS ARE SOWN
It is difficult to know where to start a historical excursion, but since a good deal of my research
life has been spent as a crystallographer I think I shall start in 1669 with the discovery by
Bartholinus in Copenhagen of the phenomenon of double refraction in Iceland Spar. This remains
for me one of the most fascinating of natural phenomena and I keep a small crystal - picked up
for a few francs in the flea market in Paris - on my desk for the sheer pleasure of seeing whenever
I wish the remarkable doubling of the image. But, of course, it is very much a key discovery
in piecing together our ideas on the transverse nature of light - though these deductions did not
follow till much later.
My next milestone must, I think, be Grimaldi’s work on diffraction. He was only 45 when he
died in Bologna and his major work was published in 1665 - two years after his death. His
experiments resembled Newton’s experiments using a prism to analyze a beam of sunlight coming
through a shuttered window into its spectral components. But Grimaldi used only a pin hole or a
slit in the shutter and observed that, as the hole was made smaller, or the slit narrower, the bright
patch at first decreased in size but subsequently grew larger again and that the dark edges became
coloured.
Newton first published some of his ideas on light in 1672 but his major work - Opticks [3]
did not appear till 1704. Fortunately, it is available in a modern reprint form and well repays
study. I can remember being guided to dip into it by my physics teacher at school when I was
only about 16 and being completely fascinated by the clarity and thoroughness of his descrip-
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