# Light momentum inside a dielectric medium?

1. Dec 13, 2007

### Repetit

Light in a vacuum has momentum $$\hbar k$$ but when the light enters a dielectric medium, fx. glass, the momentum of the light is increased to $$\hbar\sqrt{\epsilon} k$$, where $$\epsilon$$ is the dielectric constant of the dielectric. Where does the extra momentum come from? How can this be in accordance with conservation of momentum?

Thanks

2. Dec 13, 2007

### kanato

I would have argued (rather naively btw) that the momentum is $$hv/\lambda$$, and since the wavelength and wave velocity both decrease by a factor of $$\sqrt{\epsilon}$$ that the momentum is not changed. How did you get that $$k$$ increases by $$\sqrt{\epsilon}$$?

3. Dec 13, 2007

### clem

There is reflection at the boundary. Conservation of energy requires including the reflected wave. Conservation of momentum need not follow, because the dielectric material can acquire momentum.