focal length of converging glass lens in water

Question

A converging glass lens (n = 1.52) has a focal length of 40 cm in air. Find its focal length when it is immersed in water, which has an index of refraction: nw = 1.33.

Answer 1

relative refractive of glass index in water = ng/nw = 1.1428

 

Formula associating focal length with Radii of two surfaces: 1/f = (n-1) (1/R1 - 1/R2)

 

  (1/R1 - 1/R2) is constant for a lens,

 

f2/f1 = (n1-1)/(n2-1)

 

Here n1 = 1.52, n2 = 1.14286, f1 = 40cm, f2 = ?

 

f2 = f1 * (n1 - 1) /(n2 -1) = 40 *.52 /.14286 = 145.597 cm