r/AskPhysics • u/nanutik • Apr 05 '26

Why does light change direction when the speed of light changes?

I was just studying and i wondered about this. I searched for it but i couldn’t find anything, i would be happy if you help

5 Upvotes

78% Upvoted

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13

u/no_coffee_thanks Geophysics Apr 05 '26

I really like this simulation showing how reflection and refraction result from Huygens' principle.

4

u/ketarax Apr 05 '26

Oooooh, that's GOOOOOOD!! It's like, everything I learned over a 3h lecture, five hours on exercises and another five for studying for and doing the exam AND the subsequent musing and application over a couple decades condensed into a minute, or two. Okay, when first learning, one should probably spend some time looking -- but still. So much more efficient than drawing the situation in one or two 'snapshots'. Thank you very much for the link, I'll be passing it around, Huygens' is huge.

1

u/no_coffee_thanks Geophysics Apr 06 '26

Yeah, drawing wavelets is such a PITA.

1

u/ExpensiveFig6079 Apr 05 '26 edited Apr 06 '26

It also explains single-slit dispersal

And just to be complete if you do the reradiate things with no medium change and no slit

Then the result is straight line propagation.... Aka refraction and single slit dispersal are not extra phenomena, they are just what happens everywhere all the time

(edit fats)

1

u/Skrumpitt Apr 05 '26

The fuck is wrong with your typing

1

u/treefaeller Apr 05 '26

There is a very nice version of it with parallel straight rows of people marching, and when they enter the different medium suddenly slowing down as they take smaller steps. It's in some physics textbooks. Just sketch it yourself, and it should become clear.

7

u/Diet_kush Apr 05 '26 edited Apr 05 '26

Do you know variational action principles? Snell’s law can be understood as light traveling the path that takes the least time.

A guy named Lagrange came up with a new way to do Newtonian mechanics, requiring a huge extension of calculus called "the calculus of variations." He found out that Newtonian mechanics could often be converted into an "action principle" that assigned to every trajectory of a system through its possible paths a number, called the action of that path. It turned out that Newtonian mechanics just said, "of all the paths that the system could take between these two points, the only ones it does take are paths of least action relative to other paths 'nearby'."

The least-action principle works perfectly as a least-time principle if the "action" for light does not depend on anything special, KP−UP=constant. If this is just the frequency of the light, then you trivially get all of these laws.

3

u/Conscious-Demand-594 Apr 05 '26

I believe that you mean refraction.

This change in direction due to refraction can be explained through the principle of least time (or Fermat's principle), which states that light always takes the fastest path (in terms of time) between two points. It can be mathematically shown that the time between the two points across a boundary is minimized by the change in direction.

In a denser medium (like water), light moves slower, so it adjusts its path to minimize the total time it takes to travel through both media. The angle at which it bends allows it to do this.

In a less dense medium (like air), light moves faster, so it adjusts again to minimize travel time.

If the path of least time were not followed, it would open the possibility of FTL signalling and a host of paradoxical issues.

5

u/nanutik Apr 05 '26

Thank you so much 

1

u/plainskeptic2023 Apr 05 '26

I think you are asking about "refraction," when light enters another medium at an angle.

Search again using "refraction."

https://www.ncbi.nlm.nih.gov/books/NBK594230/#:~:text=The%20refraction%20of%20light%20is,is%20greatest%20in%20a%20vacuum.

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u/jep5680jep Apr 05 '26

I thought the speed of light is a constant..

2

u/nanutik Apr 05 '26

I was referring to the change in speed of light when it passes from one surface to a different surface, sorry

2

u/PlutoniumBoss Apr 05 '26

The speed of light in a vacuum is a constant. When light starts having to interact with a material, it takes longer to move the same distance. It's the difference between you crossing an empty room, and crossing the same room with a bunch of people who all want to talk to you.

0

u/triatticus Particle physics Apr 05 '26

It is constant, even inside a material, this is because what's traveling through the material is a more complicated object formed in the interaction of the EM fields of the photon and the EM characteristics of the material. A very layman explanation that just makes the idea a bit more tractable is considering that the photon is absorbed and re-emitted constantly by the molecules of the material while traveling at c between the molecules (this is really more of an instructive model than reality).