When light passes from a medium with less refractive index to a medium with higher refractive index, it bends towards the normal. But why normal . Of course we can take an example of a car moving from the road to the mud in an angle. Its first tire decides the direction. But what about light . It's not a car. It's way small and way too fast.


This is due to Fermat's principle, which states that light travels between two points along the path that requires the least time, as compared to other nearby paths. Light travels more slowly in a denser medium, and hence will bend more toward the normal.

Why then does light follow this path, and not other paths? This is because light also obeys the principle of least action, and the action for light is proportional to the time for which it travels. Thus, least action requires that the time taken be minimal.

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  • $\begingroup$ IMO this would be a better answer if it didn't present Fermat's principle as if it were the only possible answer to this "why" question. There are many other possible ways to get at this, including wave kinematics or a description using Maxwell's equations and the dielectric properties of a medium. $\endgroup$ – user4552 Sep 8 '18 at 15:01
  • $\begingroup$ @Ben Crowell: I am aware of that, but I do not know the level of knowledge of the questioner, so I chose to start with something simple. $\endgroup$ – user7777777 Sep 9 '18 at 5:07
  • $\begingroup$ If light chooses the shortest path, why didn't it choose the normal itself? And also, when passing from denser to rarer medium why doesn't it choose the shortest path $\endgroup$ – Adhesh Sagar Sep 9 '18 at 18:15
  • $\begingroup$ @Adhesh Sagar: The light does not take the shortest path, it takes the path of least time. The light is trying to reach your eye; this is how you see in the first place. See this post, it has already done a great job explaining Fermat's principle. $\endgroup$ – user7777777 Sep 10 '18 at 3:41

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