Colours

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.

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.

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.

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 -

Polarization Powdering

Rainbow

Rayleigh Scattering

Synchrotron Radiation

Thermal Emission

Least Time Principle

Mirages

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.

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.

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.