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

Colour
Colour
R.D. EDGE.
Colour, like pitch in music, is a perceptual quantity - what we see, light from objects, is observed
by the human eye and interpreted by the brain. Physics represents only a part of this, but it is the
part most easily dealt with. Nevertheless, teaching art students and other non-physicists the
physical basis of colour and its relationship to the appearance of pigments, colour film and colour
television presents no easy task - an artist perceives colour in quite a different way from a dye
chemist or a physicist. Because colour overlaps the provinces of physics, psychology and engineering,
the terminology used is mixed and confusing. Emphasis wil be laid on these differences to
try to avoid this problem.
LIGHT
Thomas Young first demonstrated light to be a wave phenomenon in 1801 by showing that the
troughs and crests of two overlapping beams of light can interfere to produce visible dark and
bright bands. In 1864 Maxwell showed light to be a form of electromagnetic radiation. Long
before Newton’s Opticks [ 11 , it was known that white light could be spread into a spectrum of
colours by a glass prism. The actual range of wavelengths which can be seen is small - from 360
to 740 nm (1 nanometre = lo-’ m), less than a factor of two, an octave. Although we shall
describe light as a wave in our discussions of colour and in its path from the source to the eye,
modern investigations show its production and detection can often be more easily described by
particle behaviour - the ‘photon> of light. Once we have split the light emitted by, or reflected
from, an object into its component wavelengths, as by a prism, and measured how much of each
there is, thus obtaining the intensity spectrum, we have virtually completed the physical specification
of that light - there is little else to discuss. (We are not interested in polarization - the
vibration direction of the light.) However, the colour which we perceive wil also depend on the
surroundings of our object, how bright the illumination is and other factors.
Light is energy, and as such may be converted into other forms of energy - such as heat (as
we notice, sitting in the sun) or electrical energy, as by a photocell. We are interested in the
illumination of objects and wish to measure the flow of the light energy onto objects we observe.
The radiance, or flow of light energy per unit area (sometimes called the flux) is measured in
physical units of watts per square metre, the watt being a joule per second.
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