Focal length of combined glass lens and water lens

[desmond iking]

Homework Statement

please refer to the photo,

Homework Equations

The Attempt at a Solution

why can't 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

 

[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.

 

[desmond iking]

understand it now finally!