`Inever approve, or disapprove, of anything now," says Lord Henry Wotton in The Picture of Dorian Gray. "It is an absurd attitude to take towards life. We are not sent into the world to air our moral prejudices." Shortly afterwards, his little mind untroubled by the hobgoblin of his inconsistency, he continues: "A cigarette is the perfect type of perfect pleasure. It is exquisite, and leaves one unsatisfied. What more can one want?"
Lord Henry's enthusiasm is a relic from a bygone age - cigarette smoking nowadays has virtually taken over from other activities the role of "the love that dare not speak its name". But observed from a safe distance, the smoke from a cigarette has much to tell us about the optics of the atmosphere.
Cigarette smoke, as we know, is usually blue.
It consists of very tiny particles of solid matter, but in so far as these little motes are possessed of any hue, it is not blue; the blue we see is merely the effect they have on rays of light that strike them.
Light is a sequence of electromagnetic waves to which our eyes are sensitive. It acquires its colour from its wavelength; blue light has a very short wavelength, "red waves" are long, and the other colours of the spectrum fall somewhere in between. Sunlight is a mixture of them all - we call it "white".
Left to its own devices, light travels in a straight line through the air. But if its path is littered with obstructions - like smoke particles of roughly the same size as the waves of light themselves - the light is scattered ; it is deflected from its original track.
Now it so happens that the particles of smoke rising from the lighted end of a cigarette are similar in size to the wavelength of blue light.
Blue light is therefore scattered more effectively than the other colours of somewhat longer wavelength, and when we look at the rising plume we see, not the tiny particles of which it is composed, but the blue portion of the sunlight scattered back in our direction by the smoke.
When a smoker has inhaled his smoke and expelled it from the mouth, each particle has become bigger than it originally was - either by absorbing moisture itself, or by having a droplet of water form upon it as a nucleus.
These particles, being bigger, scatter not just the blue light, but the light of all the other colours, too - a mixture which makes the exhaled smoke look white.