Several shades of smoke

A plume of smoke from a fire or a cigarette appears as a bluish tinge, but seen against bright sunlight - as in the case of smoke…

A plume of smoke from a fire or a cigarette appears as a bluish tinge, but seen against bright sunlight - as in the case of smoke emerging from a chimney and viewed in the evening against a cloudless western sky - it appears to be a dirty, yellowish brown. Yet smoke per se is almost colourless.

Usually it consists of very fine particles of solid matter which we cannot see as individuals; its hue is not an inherent characteristic of the substance, but depends on the way in which it is illuminated.

Light 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, and we call it "white".

Left to its own devices, light travels in a straight line through the transparent 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.

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Often the particles of smoke are of such a size that they scatter blue light very efficiently, but the red and orange wavelengths are relatively unaffected. So when we look at smoke with the light behind us, we see, not the particles of smoke themselves, but the blue portion of the sunlight selectively scattered backwards in our direction by the particles of the pollutant.

Things however are different when we look at smoke against the sun. Now we see light which has passed through the cloud of particles and, in this case, much of the blue has been extinguished by being scattered or deflected elsewhere. The residual mixture of red, orange and green adds up to a colour with a distinctly yellowish tinge.

Sometimes it happens that the solid particles acquire a modicum of moisture - as, for example, when cigarette smoke is inhaled and then exhaled or when moisture from the atmosphere condenses on the tiny nuclei.

This has the effect of making each particle significantly bigger, so that collectively they scatter light effectively in all the visible wavelengths. Therefore, the light scattered back in the direction of the eye in these circumstances is a mixture of all the colours of the spectrum - and appears as white.