PHD Discussions Logo

Ask, Learn and Accelerate in your PhD Research

Question Icon Post Your Answer

Question Icon

1 year ago in Electromagnetism , Optics , Physics By Govind

How does the speed of light increase when it enters a rarer medium from a denser medium?

In my electromagnetics class, we use the refractive index n = c/v and Snell's Law, which shows the speed v increases as n decreases. But the textbooks often stop at the math. I'm struggling with the physical picture: if the photons are the same, what physically changes in the medium to allow the wave to propagate faster? Is it a reduction in interaction delay?

 

All Answers (1 Answers In All)

By Vishal Answered 1 year ago

From my experience teaching this topic, the confusion often lies in thinking of light as just particles. Think of it as a wave interacting with a sea of bound electrons in the material. In a denser medium, there are more electrons per volume. The incoming electric field forces these electrons to oscillate (polarize), and their re-radiated fields combine with the original wave. This interaction process delays the progression of the wave crests the phase velocity slows. In a rarer medium, there are fewer electrons to polarize per unit volume, so this delay is less pronounced, and the wave crests can advance more quickly. The photons' energy doesn't change, but the collective motion of the wavefront does.

 

Your Answer