optics - Does light loses its energy when it passes through denser medium?

I know it does not because it emerges out of denser medium at 300,000 KM per second, but according to $E=mc^2$ and given that speed of light decreases inside denser medium with refractive index greater than 1, does not it suggest that energy of light inside denser medium is less?

Answer

$E=mc^2$ is not really applicable to light. It is applicable to something that has mass.

The energy of light is given by $E=h\nu$ where $\nu$ is the frequency of light and $h$ is Planck's constant, which has a value of $\approx 6.626 \times 10^{-34} J.s$

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.