# Binoculars, water

1. Aug 20, 2013

### usn7564

1. The problem statement, all variables and given/known data
A type of binoculars used for observing lobsters 10m below the water surface has an ocular with f = 40mm and an objective with f = 200mm (on the side of the air). The objective is a plano-convex lens. The tube is filled with air. What's the distance between the lenses?

2. Relevant equations
n_water = 1.33
n_glass = 1.52

1/f = 1/s + 1/s'

$$\frac{n_a}{s} + \frac{b}{s'} = \frac{n_b - n_a}{r}$$

3. The attempt at a solution
Tried playing around with the lensmaker formula but not getting past the radius which I presume I can get from f = 200mm, but not quite sure how the focal length is affected by the fact that there are several surfaces.

Last edited: Aug 20, 2013