i have a question that'll really test your minds...or not.... Im still just getting to grips with some of this physics stuff but i cant work this out. If light travels through an object it takes longer to pass through than in a vacuum right? now if heard this is because the photons are absorbed and reemitted by each particle of matter that they encounter. However this would mean that their momentum must somehow be conserved (the photons all carry on is the same direction until they leave the object). How is this 'direction' conserved? my problem is that ive been taught that momentum is the product of mass times velocity. but if a photon has no mass it cant then have momentum! im sure its explainable and that i just dont understand enough quantum mechanics yet to answer it myself. If you think you can help, post away!?!?