Nineteenth century settlers sinking wells for water in Pennsylvania were, now and then, unpleasantly surprised. A spring would sometimes be contaminated with a thick, black, oily substance.
However, despite its ugliness, the arrival of the internal combustion engine made oil an indispensable commodity.
Black oil mixed with water is still as ugly as it seemed to those Pennsylvanians many years ago. It congeals into large and viscous tarry globules to pollute a golden beach, and sometimes, as we have seen with the Claddagh swans in recent times, it causes cruel damage to indigenous wildlife.
In smaller quantities, though, oil can draw some very pretty pictures - concentric multicoloured patterns embellished with all the rich flamboyance of the rainbow. They arise from an optical process known as interference. Light, being a form of electromagnetic radiation, propagates itself in tiny waves - like those on a windy sea but smaller and moving much more rapidly. When a ray of light hits a thin film of oil at an appropriate angle, part of the light is reflected towards our eyes.
Some, however, penetrates the film and is reflected from the lower surface. This then has to travel a slightly longer path before it reaches us - the extra distance being approximately twice the thickness of the film. These two separate components of the reflected light may sometimes "interfere" with one another.
The light waves started out nicely in phase, with crests and troughs lined up in an organised way one after the other.
However, after reflection - if, for example, the difference between the distance travelled by the two components is just half a wavelength of a particular colour light - then light waves proceeding towards the eye will be out of phase; the crest of a wave from one component matches the trough of the other, so that they tend to cancel each other out.
As a result of this mutual annihilation, light of this wavelength will be missing from the composite reflection; that which reaches the eye is a combination of the colours which are left. Interference can occur with any very thin film whose thickness is comparable to the wavelength of light. It can also be seen with soap bubbles and very thin cellophane.
Since the process depends on the length of the extra path travelled by light reflected from the lower surface - and hence on the thickness of the film itself - the coloured patterns are contours of constant thickness of the film, and their distorted movement conveys to us a sense of the currents and eddies present in the oily liquid.