Super colour fractal optics in expert doses

Colour and Light in Nature
February 9, 1996

The role of colour and light in nature and, more particularly, atmospheric optical phenomena, are given scant attention in physics and meteorology courses at any level. This is a pity, because textbooks on physical optics are often rather dull and would be greatly enlivened by drawing on natural manifestations of reflection, refraction, dispersion and scattering which this handsome book illustrates and explains so clearly and well.

The main attraction is the remarkable collection of cleverly chosen colour photographs, some of which are stunningly beautiful, enhanced by the excellent layout, typography and line diagrams. The text, though generally clear, is occasionally marred by infelicities of style and expression, inaccuracies in definition and spelling, so that it does not quite match the high quality of the illustrations. The explanations, likewise, are generally accurate but sometimes rather inadequate in that phenomena such as polarisation, extinction, and optical depth are discussed but not defined.

The first chapter deals with shadows, perhaps at too great a length in comparison with other topics, but contains an incredibly beautiful photograph of the shadow of a high mountain, Kitt Peak, as viewed from the summit at sunset. This is my choice for best photograph in the book.

The second chapter, headed "Clear air", describes phenomena resulting from the scattering and absorption of light by the atmosphere and by particles suspended in it. There are clear, if rather simplified, explanations of twilight phenomena illustrated by excellent photographs of the earth's shadow, the twilight arch, purple glow and alpine glow. Manifestations of atmosphere refraction, notably of the various forms of mirage, are well described but, for once, the ray diagrams are rather unhelpful and may confuse the general reader.

It is surprising that mirages are given seven pages whereas aurorae, the most spectacular of all atmospheric phenomena, are given only half a page, and one, almost monochromatic, photograph. (As a matter of fact, aurorae are not best seen near the geomagnetic poles but at 60 to 70 degrees latitude).

Despite many attempts, I have never seen the elusive green flash nor met anyone who has. However, I am convinced by the spectacular photographs on pages 48-49 and by the explanation that it generally lasts for only a few seconds just as the sun's disk disappears below the horizon as the result of vertical dispersion of the light from its upper rim.

The chapter on water and light describes phenomena resulting from the reflection and refraction of light at the surface or the bottom of a lake and from the scattering of light by suspended particles. Some of these are commonplace, but others that arise from the interaction of the light with waves on the surface are spectacular and rather difficult to explain.

Good examples, beautifully illustrated here, are the "sky pools", which appear as elongated blue ovals on the rippled surface of a lake surrounded by distorted images from the opposite shore. They are caused by reflection of sky light from different parts of the wave profile; their width depends on the wavelength and slope of the wave and the distance of the observer from it. Complementary "land pools" may be formed, on the wave slopes facing away from the observer. Intermingling of the two produces spectacular, evanescent patterns.

Photographs in the chapter devoted to rainbows and related phenomena, though not the most striking on record, are well chosen to complement the explanations of primary, secondary and supernumerary rainbows. One photograph that captures simultaneously a conventional rainbow and one formed by sea spray is probably unique. The angular width of the latter is the smaller by 0.8 degrees because of the slightly greater refractive index of sea water. There follow discussions on solar and lunar coronael, iridescence in clouds and glories, all of which are produced by the diffraction of sunlight by clouds or fog consisting of small water droplets.

The "Brocken spectre", a particular favourite of meteorologists, is formed when the sun casts a shadow of the observer on a uniform patch of cloud, which surrounds the shadow by a coloured diffraction halo. It can best be seen from an aircraft, when the shadow and accompanying halo may follow the aircraft for miles. This phenomenon is very difficult to photograph, which accounts for the three examples on pages 131-132 being rather faint and fuzzy.

The most comprehensive and coherent chapter in the book treats a wide variety of halo phenomena produced by the refraction and reflection of sunlight by clouds composed of ice crystals. Besides the common 22o halo, illustrations and explanations are presented of the rare 46o halo, of the parhelic circle, tangent and lateral arcs, circumzenithal and circumhorizontal arcs, parhelia (mock suns), sub-suns and sun pillars, all in terms of the shapes and orientations of ice crystals that usually exist as hexagonal columns or plates, depending on the cloud temperature. Small crystals, and those of similar length and diameter, are equally likely to fall in either of two different orientations to produce a multi-halo display containing several of these elements.

The last chapter, of 50 pages, entitled "Naked-eye astronomy", reflects the professional interest of the authors, who are astronomers, but it seems out of place in that it is neither relevant to the main theme, nor substantial on its own account.

Even so, and overall, this is a most attractive and instructive book that can be read with profit and pleasure by anyone with a good grasp of A-level physics and also, perhaps, by artists, many of whom pay too little attention to light, shadow and colour in the landscape and paint only notional clouds that contain little of the subtle illumination, form and structure of real clouds. The hardback edition of Pounds 40 will make a handsome present; the paperback version at Pounds 17.95 is a real bargain.

Sir John Mason was director general, the Meteorological Office, 1965-83, and is chancellor, University of Manchester Institute of Science and Technology.

Colour and Light in Nature

Author - David K. Lynch and William Livingston
ISBN - 0 521 43431 9 and 46836 1
Publisher - Cambridge University Press
Price - £40.00 and £17.95
Pages - 254pp

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