The scattering of light by the individual particles in smokes

Theoretical investigations made by Rayleigh on the scattering of light have shown that for indefinitely small spherical particles the intensity of the light scattered by individuals should be proportional to the sixth power of the radius. These investigations have been extended by Mie to spherical particles whose diameter is comparable with the wave-length of light, and he has found that, although Rayleigh’s Law should hold good for particles whose diameter is less than about one-tenth of the wave-length of light, for larger particles the power of the radius will gradually decrease with increase of size to less than two, and will then increase, becoming two again when the particle is sufficiently large to act as a true reflector. These calculations can only be applied accurately to individual particles or to dilute systems, as otherwise the calculations may be invalidated by multiple scattering. No practical investigations have hitherto been made on individual particles. The few experiments which have been carried out have always been concerned with the average scattering per unit volume of comparatively condensed and inhomogeneous systems of particles, and it is doubtful whether the results are free from the complicating effect of multiple scattering. Thus, Tyndallmeter experiments have been made by Tolman and his colleagues on smokes, but the results he obtained could not be expressed in terms of any simple law of scattering or reflexion. Mecklenberg has carried out similar experiments on sulphur solutions which he believed to be homogeneous, and these indicate that Rayleigh’s Law is obeyed for the size of particles investigated.