What happens to a light wave when it meets and travels through a different medium?

Answer 1

when a wave of light of any wavelength travel in a medium at definite velocity. and

wave enter into the another medium changes in velocity due to characteristic difference of that medium with the wave of particular wavelength. The change in velocity causes the light to "bend". You can see this by standing a pencil in a half glass of water or look up convex lenses.

The mechanism is poorly understood. It is quantified by Fresnel's refractive index 'n' , where n=1 in a 'vacuum' (or ion plasma). In glass n=~1.55 so light speed slows from 186,200 to ~120,000 miles/sec.

The most consistent answer is atomic scattering. (Rayleigh/ Thompson/ Compton/ Raman scattering). Particles absorb EM wave energy and, when charged, re-emit it. (in pulses, or 'quanta' often called 'photons'). Penrose has determined that 'conserved' photons can't exist if Relativity is to be Unified with QM. (it currently is not, so physics is far from complete). They can however be absorbed and re-emitted.

The 'waves' are then Doppler shifted (red or blue shifted) due to the progressive speed change (one by one as they enter). For conditions where the new medium is moving with respect to the old, look up 'kinetic reverse refraction'. That is quite easy to comprehend, but would get a distinction at PhD level as it has not yet penetrated theoretical physics from optical science.