When electromagnetic radiation strikes perpendicular to a flat surface, a totally absorbing surface feels radiation pressure I0/c, where I0 is the intensity of incident electromagnetic radiation. A totally reflecting surface feels twice that pressure. More generally, a surface absorbs a proportion e of the incident radiation and reflects a complementary proportion, 1 - e, where e is the emissivity of the surface. Note that 0 ≤ e ≤ 1. (a) Determine the radiation pressure prad in terms of I0 and e. (b) Consider cosmic dust particles in outer space at a distance of 1.5 x 1011 m from the sun, where Isun = 1.4 kW/m2 . We can model these particles as tiny disks with e = 0.61, diameter 8.0 µm and mass 1.0 x 10-10 grams, all oriented perpendicular to the sun’s rays. What is the force on one of these particles that is exerted by the radiation from the sun? (c) What is the ratio of this force to the attractive force of gravity exerted by the sun on the particle?
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