  Reflection and refraction of light can be used to trap small dielectric
particles in the geometrical optics regime. Absorption of light is usually
neglected in theoretical calculations, but it is known that it occurs in the
optical trapping of metallic particles. Also, recent experiments with
semi-transparent microparticles have shown that absorption of light is
important to explain their optically induced oscillations. Here, we propose a
generalization of Ashkin's model for the radiative force exerted on a spherical
bead, including the contribution due to attenuation/absorption of light in the
bulk of the particle. We discuss in detail the balance between refraction,
reflection and absorption for different optical parameters and particle sizes.
A detailed example is provided in order to clarify how the model can be
applied, and it is obtained that the radiative force can either increase or
decrease with absorption, depending on the particle size. Our findings
contribute to the understanding of optical trapping of light-absorbing
particles, and may be used to predict whenever absorption is important in real
experiments.
