Correct Answer: C
Explanation:
E=mc2is not really applicable to light. It is applicable to something that has mass.
The energy of light is given by E=hν, where ν is the frequency of light and h is Planck's constant, which has a value of ≈6.626×10-34 J.s.
When light enters a different medium, its frequency remains the same, and of course, so does Planck's constant. Hence, obviously, its energy remains the same throughout the entire exercise.
E = mc2 cannot be applied in this case because the "c" in the equation is not the value of how fast the thing is travelling, it is a constant. The value of c is the speed of light in vacuum. E = mc2 applies to an object with mass at rest, hence not photons.