When light travels from a rarer to a denser medium, the speed of light decreases. Does the reduction in speed imply a reduction in the energy?

No, reduction in speed on passing from a rarer to a denser medium does not imply a
reduction in energy of light because the frequency of light remains unchanged and the energy of a light photon depends only on its frequency.
Class 12 Physics Chapter 10 Wave Optics Session 2024-2025.