I just read that, when a ray of light traveling in vacuum at $c$ strikes a glass slab, its speed decreases and then when it re-emerges it gets back to its original speed i.e $c$.
If I draw a comparison with a bullet fired at a metal sheet, then when it re-emerges its speed is drastically reduced.
So, why hasn't the same principle been applied in both the situations? Doesn't the first case violate any energy principle?
Any energy principle is not being violated since the speed of the photon is never less than $c$ and hence the momentum is unchanging (in the classical sense). Why light travels slower than $c$ in a medium is because of the photons being absorbed and reradiated by atoms in the material. In a sense you can make the analogy of light traveling a longer path in the material but retaining its speed $c$ at any given time and position, this has the same implication of conservation of energy and the speed of light in the material appearing less than $c$.
The case with the bullet is fundamentally different because then it is the speed of the particle in itself that is reduced from collision, its velocity being dictated by the conservation of momentum and energy with the interaction with the material.
