3 ms·
Conservation of momentum, I think
by melon_madness 7y ago
Conservation of momentum, I think
- whatshisface 7y agoIf the emitted photon always had the same momentum as the absorbed photon, the light would go right through the mirror, passing on its way without changing direction.
- samatman 7y agoOnly if the momentum wasn't transferred to the mirror. Which is, in fact, what happens. Solar sails exploit this fact: https://en.wikipedia.org/wiki/Solar_sail https://en.wikipedia.org/wiki/Solar_sail
- whatshisface 7y agoIf momentum was transferred to the mirror then the emitted photon would not have the same momentum as the absorbed photon.
- samatman 7y ago> Why the photon is then re-emitted in exactly the same direction that it would have if it was reflected rather than a random direction? That is, the equal and opposite direction, exactly as predicted by conservation of momentum.
- melon_madness 7y agoConservation of momentum doesn’t mean that the momentum of the emitted photon has to be equal to that of the absorbed photon. It means the sum of the momentum of the emitted photon and the mirror must equal the momentum of the absorbed photon, assuming the mirror had no momentum to begin with.
- mr_toad 7y agoMomentum is a vector. The sum of the vectors is preserved.