Radiative pressure efficiency factor Q_rp and its components Q_abs and Q_sca, relative to the corresponding small-particle limit (SPL) values Q_SPL, as functions of grain radius at λ = 1   μm (i.e., near the stellar flux maximum). Data are given for amorphous carbon dust, taken from Rouleau & Martin (1991) and the Q’s are calculated using the Mie theory for spherical particles (programme BHMIE from Bohren & Huffman 1983, modified by Draine, www.astro.princeton.edu/draine/scattering.html). The vertical dashed line marks a grain radius of 4 × 10^-6 cm where deviations in the opacity from the SPL may exceed 10% at wavelength λ = 1   μm.