  We study the absorption of plane waves by Kerr black holes. We calculate the
absorption cross section: the area of the black hole shadow at a finite
wavelength. We present a unified picture of the absorption of all massless
bosonic fields, focussing on the on-axis incidence case. We investigate the
spin-helicity effect, arising from a coupling between dragging of frames and
the helicity of a polarized wave. We introduce and calibrate an extended sinc
approximation which provides new quantitative data on the spin-helicity effect
in strong-field gravity.
