# Fermat’s principle to derive lens formula

**1. For the past 1 hour, I’m trying to derive lens maker’s equation using fermat’s principle, which of course is our homework. Any help would be great regarding the topic.**

**2. According to Fermat’s Principle, optical path length OPL = PA + AQ, here PA and AQ are two rays. Now using this I have to derive Lens makers Formula.**

**3. Now the diagrams and eqn’s i don’t know how to put in here. Upto the point where I found PA + AQ, I have completed.
PA = (x^2 + h^2)^1/2 = x + 1/2 X h^2/x
AQ = (y^2+ h^2)^1/2 = y + 1/2 X h^2/y
Now, PA + AQ = x + y + h^2/2 X (1/x + 1/y)
Now kindly help me if you happen to know the next steps.**

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