, where the free mean path of the gas is much larger than the characteristic size of the suspended particle (direct photophoresis), the longitudinal force is where the mean temperature of the scattered gas is (thermal accommodation coefficient , momentum accommodation coefficient ) and the black body temperature of the particle (net light flux , Stefan Boltzmann constant , temperature of the radiation field ) . is the thermal conductivity of the particle. The asymmetry factor for spheres is usually (positive longitudinal photophoresis). For non-spherical particles, the average force exerted on the particle is given by the same equation where the radius is now the radius of the respective volume-equivalent sphere. References External links Photophoresis in the context of astrophysics Negative photophoresis Atomic, molecular, and optical physics Physical phenomena