Indeed. To be even more specific, the reason why it is slowed down is because of absorption and re-emittance of particles of light (photons). If you go down the

Indeed. To be even more specific, the reason why it is slowed down is because of absorption and re-emittance of particles of light (photons). If you go down the the extremely small scale, what you will actually see is a photon moving at the speed of light, colliding with an atom, being absorbed by an electron, then re-emitted on the other side. Repeat this process ad nauseam and the constant absorption/re-emittance will slow down the speed of light to a varying degree. Most importantly, however, is that the c is constant in free space.