Today, my Physics teacher explained the cause of refraction by using the car analogy

It goes like this :
Suppose a car is moving on a concrete road and obliquely enters some mud. Now, one of the front tires will enter the mud earlier than the other and it will slow down earlier. So, this tire will now travel a less distance in the time in which the tire which has still not entered the mud will travel a larger distance. Due to this, the car will rotate with the tire that entered the mud as its center and its direction will change.

I get this. But I'm a little confused about how we can relate this to light while assuming that it is a wave.

An explanation would be appreciated...



2 Answers 2


Maybe this version of the same analogy is clearer for you. Instead of a car, we have a column of soldiers, say 4 soldiers wide, marching in perfect time. The column of soldiers is a better representations of the light wave than the 2 front wheels of the car, IMHO.

Now consider what happens when the column crosses the concrete / mud boundary at an oblique angle. The soldiers who step on the mud first will slow down earlier than the soldiers on the other side of the column, and the column will bend.

You don't even need a column of soldiers to achieve this rotation effect. You can feel it for yourself by trying to walk along a boundary between a firm surface and sand, with one foot on the firm ground and the other on the sand. Your body will try to twist towards the sand. This gets very annoying within a couple of minutes. ;)

  • $\begingroup$ Nice version (better)- but I think the OP was confused because the analogy deals with particles and light is a wave. $\endgroup$
    – Elendil
    Apr 27, 2020 at 11:43
  • $\begingroup$ @Krishna Exactly... $\endgroup$ Apr 27, 2020 at 13:23
  • $\begingroup$ @Rajdeep The 2 front wheels of the car are a very crude representation of the wave front, the column of soldiers is just a bit less crude. That's why I said it's another version of the same analogy. Modern physics uses both wave & particle models for light. Historically, Newton promoted a corpuscular (particle) model of light, but the success of Huygens' wave explanation for refraction and various other optical phenomena dealt the particle model a fatal blow. It took Einstein's explanation of the photoelectric effect for the particle model to be revived. Wikipedia has more details on this. $\endgroup$
    – PM 2Ring
    Apr 27, 2020 at 13:33
  • $\begingroup$ @PM2Ring Light is a Transverse wave like all electromagnetic waves, right? So, this means that the direction of oscillation is perpendicular to that of propagation. So, the wave is somewhat similar to the graph of $sin(x)$. If we have a longitudinal wave, like sound waves in air going to water, then this analogy works fine, the soldiers are the compressions and the gaps between them are the rarefactions. So, this can be used to explain why longitudinal waves like sound refract. $\endgroup$ Apr 27, 2020 at 13:37
  • $\begingroup$ But what about Transverse Waves like light and other forms of electromagnetic radiation. They have crest and trough instead of compressions and rarefactions. So, how can the analogy be applied to that? Let me know if there is something I don't know or have misunderstood about waves. I only have introductory knowledge in that. Thanks! $\endgroup$ Apr 27, 2020 at 13:38

The Car analogy I was taught is that car is faster in concrete and slow in mud, so to travel from point A(in concrete) to point B(in mud) in the least amount of time, the car have to change the direction at the junction of the two roads. The explanation you gave doesn't have to follow the Fermat's principle, which is the fundamental principle in the phenomenon of refraction of light.

  • $\begingroup$ I know what Fermat's Principle is. It does a pretty great job explaining how light will bend when it passes from one medium to another but I don't think it explains why it will bend. I thought that the Car Analogy was to explain why light bends... $\endgroup$ Apr 27, 2020 at 13:21
  • $\begingroup$ @Rajdeep_Sindhu your question didn’t ask “why” it asked “how”. If you wanted a “why” answer then you need to say so as well as explain your question in substantially more depth. “Why” questions are notoriously hard to frame in an answerable form: m.youtube.com/watch?v=Dp4dpeJVDxs $\endgroup$
    – Dale
    Apr 27, 2020 at 13:46
  • $\begingroup$ I thought that the car analogy explained why light bent as well as a bit of the rules that it followed while bending (another way to say how). I thought that the car/soldiers analogies do this by comparing light waves to wheels of a car or soldiers marching in a row and I wanted to know how to apply these analogies to light waves, indirectly asking why light waves bend by taking into account one of these analogies. And you quoted Richard Feynman, right? :) $\endgroup$ Apr 27, 2020 at 13:52

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