English: Object experiencing torque due to incident light via the Yarkovsky–O'Keefe–Radzievskii–Paddack effect (YORP effect).

The object is a black-body sphere with two wedge-shaped fins, one (A) with one face perpendicular to the light and one (B) reversed, with the front face oblique. The incident light is adsorbed and re-emitted by the surfaces.

Both fins have the same height and width, and therefore present the same area to the light (by the cosine law). As black-body surfaces, they both absorb (and re-emit) the same amount of energy.

Re-radiated light is emitted normal to the face. For fin A's light, the pressure of the radiated light is parallel to the incoming light, but Fin B's light has a vertical component, but the same magnitude, thus a smaller component paralllel to the light. Thus there is a net torque causing Fin A to move away from the light, as well as a vertical force causing the object to precess.