# Homework Help: Focal length of combined glass lens and water lens

1. Aug 18, 2014

### desmond iking

1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution
why cant I directly use the lens maker formula to get the ans ?
my working is 1/f = ( (1.5-1.33) /1.33) x ( (1/20) + 0 ) ...finally , i get my f = -160cm.
the radius of curvature of glass is 0

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2. Aug 18, 2014

### BvU

Strange, I get +160 cm using the lens maker formula as you write it.
That would be for a glass lens fully submerged in water (the 1.33 for n0)

But the exercise has a glass lens with air on one side and water on the other PLUS a water lens (concave!) with glass on one side and air on the other. In other words, a combination of TWO lenses.

The solution manual deviously inserts a thin layer of air between the lenses so that the n0 can be set to 1 for both lenses.

3. Aug 18, 2014

### desmond iking

understand it now finally!