# Fermat's principle Integral

1. Jul 24, 2014

### manimaran1605

I am unable understand this Integral, what does it actually saying? What does that "δ" means here? I haven't learnt Calculus of variations, explain me with diagrams with possible.

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2. Jul 24, 2014

### HallsofIvy

Staff Emeritus
It pretty much says what it means. The integral $\int_{A\to B} n ds$ is the integral of n along some path from point A to point B. The "$\delta$" in front means that we are considering what happens when we vary that path "infinitesimally".

3. Jul 26, 2014

### twoslit

The point that may be giving you trouble is that the integral is taken along the path, while the variation expressed by the delta is varying the path itself. Think of a path from A to B wiggling slightly: the wiggle is the delta.