Appearance of mollusk shells related to reflection by absorbing periodically stratified media.

The theory of the optical properties of an absorbing periodically stratified medium is applied to the spectral and angular dependence of the reflectivity of mollusk shells, which are almost-periodic structures of aragonite plates alternating with thinner organic layers. The reflectivity of and polarizations show strong variation as the angle of incidence passes through the stop bands of the corresponding ideal non-absorbing periodic structure. For both polarizations of incident light, and at most angles of incidence, the blue and green wavelengths are reflected more than the red-hence the blue-green appearance of mollusk shells.