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17th
October 2000 Back to Home Page
back to Physics
 
| Colour is so ubiquitous that we often
take
it for granted. More unusual in nature are black and white. Some
clouds are white, snow is white, some swans, geese and other birds are white. Coal is black, crows
and rooks are black. But the list is not long. For black and white we can look
at the magpie, pied wagtail, gannet and lapwing, marbled white and black-vieined
white butterflies, male adder, and quite a few other examples. How many of
these on closer inspection are found to be truly black and white? |
|
Black and white artefacts, too, are
not common. Some houses are black and white, for example. But
perhaps the
main use of black and white is in print, colour being the exception. We speak of "there in
black and white" and "red letter days". Black and white
seems to carry authority, so very few books are printed with coloured text or paper,
except where emphasis is needed. But in advertising, film and television,
colour is so common that black and white may be used for greater impact. |
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There are numerous ways of creating
colour. The chief division among these is between colours produced by
materials such as pigments, which work at the atomic or molecular level, and colours produced by
structures which are comparable in size with wavelengths of light, thousands of
times bigger thah atoms.
With structural
colours, the type of material
is relatively unimportant. What matters is the fine structure of its surface, or of
some layer inside it, if it is transparent, and it may show different colours
depending on the direction of the incident light and the angle of view.
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In a pigment, the substance itself has properties that make it look coloured. It has the same
intrinsic colour whatever
its size and shape, except, perhaps, when finely powdered, from any angle of view, and with any angle of
lighting. But the colour of the lighting can affect the appearance, which
is the reason for checking the colour of fabrics in daylight.
The pictures below show the same
objects illuminated by daylight, electronic flash gun, incandescent filament
lamp, and tubular fluorescent lamp. Daylight slide film was used.
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So the perceived colour of an object
depends on
several things –
1
The incident light, if the object is
not self-luminous,
2
The medium between the light source
and the object,
3
The object itself,
4
The medium between the object and the
viewer,
5
The vision of the viewer, including
the effects of optical illusions.
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The medium often has little effect,
but underwater vision and photography may be greatly affected by the absorption of
red light. Similarly, near sunrise and sunset, colours are given a reddish
bias.
Some of the mechanisms for colour
effects are –
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The raw material of colour is
light. The radiation that reaches the ground includes what people call
"visible light", which includes the continuous range of rainbow
colours from red to violet. Two other kinds are called infra red (IR)
radiation and ultra violet (UV) radiation. All these are of the same
nature except for the the length of the waves that comprise them, and the
corresponding frequencies. Ultra violet
radiation can be seen by some animals, such as bees, and many flowers have marks
which guide insects to the nectar (and pollen) – marks which are often visible
only in ultra violet light. Some snakes can detect the infra red radiation
which all warm or hot bodies emit – useful if you need to eat a mammal.
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Perceived colour is the result of
some kind of selection of wavelengths from the incoming light, and
interpretation by the eye-brain system.
The colours that you saw at the top
of the page were created by a rather complicated process, involving not only the
five mechanisms listed earlier, but also the following –
A slide film and
processing
A scanner which
produced a stream of digital data,
A program which
converted the data into JPEG format,
A cathode ray
tube or a TFT screen which interpreted the data using three coloured phosphors.
Your
own vision.
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| Explaining all the ways in which
colour is produced puts us in touch with some basic ides of science. We find a
fascinating world behind even the simplest observations. The different
kinds of science, like the different kinds of art, provide entrances to ways of
understanding that are outside the essential tasks of getting food and drink and
shelter. People have the ability to create abstract ways of thinking.
Even something so mundane as money, presumably created to avoid the
complications of bartering, has led to the complex worlds of finance and
economic theory. |
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All these abstract worlds lead to
questions which can be very difficult to answer – Where is the energy in this
system? Where is this photon? Where is the money I put in the
bank? Where is this web-site? Where is the seat of life in this
animal? Where is football?
Some people think that science is difficult, which it is – otherwise we would
all be able to do it. But it is not necessarily more abstract than many
other things that people take for granted. |
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