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

Optics regained
image ifmis-interpretation is to be avoided. (In figure 4 we saw that the restriction of the aperture
could lead to a complete misinterpretation of the number of holes in the central section.)
The importance of the human element
We see the world around us by using the eye-brain system, and it has always amazed me that so
often physics students wil learn about the eye as an optical system but will not be led to appreciate
the real significance of its being attached to a brain. Optical illusions are not merely curiosities
with which to amaze and confound your friends; they are very important pointers to the way
in which we respond to our surroundings and to the limitations that our human characteristics
impose. This section merits a whole book to itself and I can only pick out one or two examples
to illustrate the kind of thing I have in mind.
Most physicists are quite familiar with the idea of three-colour vision. Colour television is a
permanent reminder to us of its practical application. But colour is a very subjective phenomenon
and our judgement about a colour depends on circumstances, on the nature of the illumination,
on the surroundings of the colour observed and on many other factors. For example, a piece of
uniformly dyed yellow cloth which is held loosely so that it falls into folds wil be recognized
by any viewer as being of uniform colour. But if invited to look again - for example with the
eye of a painter about to paint a picture of the cloth - the viewer will realize that the retinal
image of the cloth will be made up of an enormous number of different shades of yellow. What a
remarkable process is performed by the brain in noting all these variations, in recognizing that
they fit together in a way that corresponds to the incidence of light on the cloth and to a valid
system of folds and hence deducing that the colour is in fact uniform. One of my favourite
illusions involves a large sheet of card of uniform colour - grey is useful - which is covered
entirely by another card with two square holes in it so that the viewer sees two square patches
of grey separated from each other. The upper card is divided into halves and each half coloured
uniformly with a different colour, for example, yellow and blue. The viewer thus sees each patch
of grey surrounded by a different colour and it is difficult to believe that the two patches are
indeed the same grey. The effect of the surroundings in determining the colour is very powerful.
Finally in discussing colour we might mention an effect that sometimes baffles amateur
photographers. Suppose a close-up portrait is being taken; the photographer wil carefully choose
the background, but very often wil ignore surroundings that are outside the field of view. But if,
for example, there is a red brick wall at one side of the subject, a warm red glow may be reflected
on to the face. In the live situation, as the photograph is taken, the glow is ignored because the
eye-brain system sees the wall and makes suitable compensation; but when the colour photograph
is printed the photographer cannot understand why the subject has such a red face.
We take very much for granted the ideas of perspective and happily draw pictures using the
principle that parallel lines (roads, edges of buildings, etc) wil meet at a point on the horizon.
But how often do we realize that a considerable amount of brain processing is involved in
recognizing that the lines on a flat piece of paper represent a three-dimensional object. Professor
Richard Gregory tells of a blind acquaintance of his who, as a result of an operation, had his
sight restored in later life. This man was unable to appreciate many of the common optical
illusions and had to learn from scratch how to relate images on the retina with three-dimensional
objects in the surrounding world.
Finally, in this very brief section it is worth reminding physicists from time to time that we
have a built in tendency to see what we want to see. Most people wil have had the experience
of reading an article in which there are misprints and, if the article is sufficiently absorbing, not
noticing the errors. Another reader perhaps less interested in the topic wil spot the errrors
instantly .
247