Abstract
A magnetic field splits up electron energy bands in a solid into a series of sub-bands. The absorption edge which arises from transitions between two such bands across an energy gap then shows a structure from transitions between the various sub-bands. The shape of this structure is considered for direct transitions between simple spherical bands, spheroidal bands and degenerate bands. This often takes the form of a series of peaks from which the effective masses of the bands can be determined, but there are cases in which the effect of the field is less dramatic and forms a series of steps. Absorption from indirect transitions involving phonons also forms a series of steps.