Why are solar sails reflective, not black?

It's a matter of how much momentum you transfer. For photons with momentum p0, you transfer a unit p0 of momentum to a sail that absorbs that photon. But if you use a reflective sail, the conservation of momentum requires that the sail is is pushed back by 2p0. Since the force the sail gets can be written as F = dp/dt, you get more propulsion when the sail is reflects the light rather than absorbing it.
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