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Fermat's Principle 
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#1
Jul2414, 05:58 AM

P: 58

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. 


#2
Jul2414, 06:18 AM

Sci Advisor
Thanks
PF Gold
P: 12,251

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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