What happens to light as it enters a denser medium? I am a first year undergrad student doing optometry (never done any physics before in my life :( ). I got a question asking what happens when light enters a denser medium. I was told that the frequency of light doesn't change, but the wavelength decreases. Is that true? And if so, why does the frequency never change? As little maths as possible would be great - this subject is mostly very qualitative and I'm not a maths expert.
 A: Frequency depends upon source. It is just the number of peaks or troughs passing though a point each second.
Imagine you are holding one end of a rope and other end is tied to a wall and you are oscillating your hand up and down.Now the number of peaks passing on the rope through a point per second just depends on how you oscillate your hand. Unless you change the frequency of oscillation of your hand,the frequency of wave on rope won't change.
This is similar to the case of electromagnetic waves as well.Their frequency won't change in different mediums unless the source is changed.
Meanwhile wavelength decreases in a denser medium(its refractive index is high) as it travels slowly in it.
A: "I was told that the frequency of light doesn't change [...] Is that true? And if so, why does the frequency never change?
It is true. Frequency is the number of cycles of oscillations per second. Each cycle of oscillation is passed through the medium and from medium A into medium B and so on. You can't lose or gain cycles of oscillation on the way. If you could lose cycles, ask yourself: which ones? Every other cycle, every fifth cycle? Random cycles? It really doesn't make sense! Nor could you gain cycles.
Wavelength does decrease when light passes into a medium in which it travels more slowly. In the time for one oscillation the wavefronts don't travel as far.
A: Light gets absorbed and re- emitted more in the denser medium and therefore  takes longer to pass through it
