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B Simple proof of Snell's law without calculus

  1. May 10, 2017 #1
    Well, I have checked out the ones with calculus but I was just wondering if there was one without calculus
    I tried it but could not do it
    I think Fermat's principle can be used to do it but I am not being successful
    So, anyone please help
  2. jcsd
  3. May 10, 2017 #2


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    Yes. I've found it on Wikipedia, but I do not know what you mean by "without calculus". I even don't know, how to formulate it without calculus.
  4. May 10, 2017 #3
    No surprise since Fermat's principle is a type of variational principle, so it is somewhat connected to calculus.
  5. May 10, 2017 #4


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    This question doesn't make any sense. It is like asking if one can do kinematics with something that has variable velocity and acceleration but without calculus. In many of these cases, not using calculus is NOT an option!

    And as stated already, Fermat Principle itself is based on calculus, i.e. finding the path of least time, which is a variational calculus application (see Pg. 8 of this document). So I have no idea why you'd think that using Fermat Principle is not using calculus.

  6. May 14, 2017 #5
    Oh yes it does.
    But it can be done WITHOUT calculus too.

    You can prove snell's law too WITHOUT calculus.

    I have seen the proof and it is by R.P.Feynman.
    So, thanks but no thanks
  7. May 14, 2017 #6


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    Feynman's "proof" isn't really a proof. At least the example I've seen from his lectures isn't. It explains how you can use geometry to find an answer in a specific case where there's a single boundary and two non-varying refractive indices. A real proof inherently requires calculus because it has to deal with continuously varying variables. Page 8 in Zz's link contains this exact situation and you cannot use geometry by itself to solve it.
  8. May 15, 2017 #7
    Can't you use the property of isotropy of space and the time invariance of physical law? But that may only prove that incident angle is equal to the reflected angle for total reflection.
  9. May 15, 2017 #8


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    since you are going against good advice and say it can be done
    then the onus is on you to show us how it is done and we can see if it makes sense
  10. May 16, 2017 #9


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    You only have to match the wavevectors parallel to the surface of the light inside and outside the medium. This doesn't involve calculus, merely vector arithmetics.
  11. Oct 15, 2017 #10
    But when fermat devised his principle at that time there was no calculus. As calculus came much after by newton and leibnitz.Fermat must have done in his own way.Or he just gave us an intuitional principle with no proof or he had find maxima or minima in his own way.

    I gues this question/topic needs importance.


    Moderator's edit: File substituted by link due to potential copyright violation.
    Last edited by a moderator: Oct 16, 2017
  12. Oct 15, 2017 #11
    Being an Indian,perhaps you have gone through H.C.Verma .There also you will find a simple but elegant proof of the above.
    I did not post anything since by the time I started the thread the book has been returned and I could not include the proof on the site .
    Thanks once again.
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