# Homework Help: Focus of Lens Submerged In Water

1. Aug 30, 2011

### JSGandora

1. The problem statement, all variables and given/known data
A glass lens (n=1.50) in air has a power of +5.2 diopters. What would its power be if it were submerged in water?

2. Relevant equations
Not too sure. May be Lensmaker's Equation, Snell's Law, and Power of a Lens Equation.

3. The attempt at a solution
I'm trying to derive the Lensmaker's Equation with the surrounding substance having index of refraction 1.33 but that didn't work. I also tried changing the index of refraction of the lens so that it was with respect to the speed of light in water rather than in air.

2. Aug 30, 2011

### tiny-tim

Last edited by a moderator: Apr 26, 2017
3. Aug 30, 2011

### JSGandora

Haha, I found it.

I have found the solution to be 1.3 diopters. You just divide the refractive index of the lens by that of the water to get the effective refractive index from Snell's Law. That derivation of the Lensmaker's equation holds true so we get $\left(\frac{1.5}{1.33}-1\right)(10.4)\approx \boxed{1.3D}$. I got the same concept last night without knowing that it was correct because I didn't subtract 1 from the ratio of refractive indices. >.<