On the Optics of Small Graphite Spheres, II

Mie computations are performed for small graphite spheres using the model refractive index given in Paper I. The efficiency factor Q _ext and the albedo γ are computed as functions of wavelength for particles of radii between 0.010 µ , and 0.065 µ . For radii less than ∼0.03 µ the extinction follows an approximately λ⁻¹ law in the range 1.3 µ > λ > 0.45 µ but begins to rise more steeply for shorter wavelengths. Normalized extinction curves are constructed for Gaussian distributions of particle sizes with small dispersions centred at $$\bar a=0.015\enspace\mu, \enspace0.020\enspace \mu,\enspace0.025\enspace\mu$$ . Agreement with the observational curves remains generally tolerable for wavelengths longward of 4 500 Å, but the theoretical curves show excess extinction towards the ultra-violet. For a grain radius a = 0.02 µ the ratio Q _ext (2 000 Å)/ Q _ext (4 000 Å) ≅ 7, but this ratio decreases with increasing radius. Good agreement with the observational curve is obtained for an Oortvan de Hulst distribution of graphite grains with a mean size $$\bar a=0.028\enspace\mu$$ .