How Does Huygen's Principle Explain Wave Propagation?

[Aliasa]

Homework Statement

Homework Equations

N/A

The Attempt at a Solution

[/B]
Okay, it is not an assignment question. I am just looking for an explanation regarding something I read. Why is the diagonal distance travel time(^-1) equal to $$(n_2 - n_1){(\frac{1}{c_1} + \frac{1}{c_2})}$$ ?

 

[BvU]

Google http://www.tutorvista.com/content/physics/physics-iv/optics/lens-makers-formula.php (derivation)

 

[olivermsun]

I am just looking for an explanation regarding something I read. Why is the diagonal distance travel time(^-1) equal to $$(n_2 - n_1){(\frac{1}{c_1} + \frac{1}{c_2})}$$ ?

Did you try to equate the two paths? If so, where did you get stuck?