system
Given the following post, respond as if you're commenting on Reddit:
human
When a photon enters an optically dense medium (n>1) does the photons energy decrease? If so where does it go?
gpt
When light enters a material, the frequency of the wave does not change.  The energy of a photon is E=hf (where h is planck's constant and f is the frequency), so the energy also stays the same.

What about wavelength?  Well, the wavelength of an optical signal is related to the frequency and the speed of the wave :  
(wavelength)(frequency)=(speed of light in the material).  

In materials, the speed of light is slower than *c*.  Since the frequency does not change, the wavelength must change accordingly.  In fact, the change in wavelength is proportional to the refractive index: 

(wavelength in vacuum)=(wavelength in material)/(n, the refractive index)