Colour from scattering

Introduction

Many of the colours in nature, particularly - but not exclusively - the blues, are the result of the physical process of scattering rather than the presence of a chemical pigment. For scattering particles with a size comparable to the wavelength of light (about 0.5 micron), there is a strong dependence of scattering efficiency (cross section) on colour and the balance between the light scattered into and out of the beam entering our eye will determine the hue we perceive. The larger particles, eg. water droplets in a cloud or ice crystals in snow, give rise to some of the most brilliant whites.

In animals a whole range of blues, greens (blue + yellow pigment) and whites are produced by scattering and blue pigments are rather rare. Other blues, usually of a less subtle hue, can be produced by interference and diffraction effects - as in the 'Morpho' butterfly. In biology, the process which produces the blue colours is usually referred to as "Tyndall" scattering afer John Tyndall who discussed the blue sky in 1869 although the term "Rayleigh" scattering is usually used in physics.

The colour of the sky

The blue of a Jay's feather

Scattering in solids

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