Electromagnetic radiation, motion of a particle and energy-mass relation
  Equation of motion of an uncharged arbitrarily shaped dust particle under the effects of (stellar) electromagnetic radiation and thermal emission is derived. The resulting relativistically covariant equation of motion is expressed in terms of standard optical parameters. Relations between energy and mass of the incoming and outgoing radiation are obtained, together with relations between radiation energy and mass of the particle. The role of the diffraction nicely fits the relativistic formulation of the momentum of the outgoing radiation.   The inequality 0 < $\bar{Q}'_{pr, 1} / \bar{Q}'_{ext}$ < 2 is a simple relativistic consequence for the Poynting-Robertson (P-R) effect ($\bar{Q}'_{ext}$ and $\bar{Q}'_{pr, 1}$ are dimensionless efficiency factors for the extinction and radial direction of the radiation pressure, integrated over stellar spectrum). The condition for the P-R effect is $\vec{p}'_{o}$ = (1 - $\bar{Q}'_{pr, 1} / \bar{Q}'_{ext}$) $\vec{p}'_{i}$, where $\vec{p}'_{i}$ and $\vec{p}'_{o}$ are incoming and outgoing radiation momenta (per unit time) measured in the proper frame of reference of the particle. The case of "perfectly absorbing spherical dust particle", within geometrical optics approximation, corresponds to the condition $\vec{p}'_{o}$ = 0.5 $\vec{p}'_{i}$. As for arbitrarily shaped dust particle, the condition 0 < $\bar{C}'_{pr, 1}$ / $\bar{C}'_{ext}$ < 2 / ($1 ~+~ \sum_{j=2}^{3} \bar{C}'_{pr, j} / \bar{C}'_{pr, 1}$) holds for cross sections of extinction and radiation pressure components. The condition can add a new information to the results obtained from observations, measurements and numerical calculations of the optical properties of the particle. 
J. Klacka