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Fermat's Principle expression

  1. Jul 24, 2014 #1
    The expression gives time taken to reach a a distance ds in a medium.
    The expression has the dimension of time, But my question is how come the denominator is (c/n)? why it can't be (cn)? explain me.

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  3. Jul 24, 2014 #2


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    Bearing in mind that the speed through a medium must always be less than c and that refractive index is always specified such that it is greater than unity. Using c/n produces the sensible answer, which is that light will always take longer to travel through a medium than through a vacuum.
    IF n were defined the other way round, then your alternative would apply.
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