The radii of curvature of a lens are +20 cm and +30 cm . The refracting index of the lens material is 1.6 . Find the focal length of the lens if it is placed in water ( = 1.33 ).

Step-by-step solution

Using the lens maker's formula: 1 f = ( _l _m - 1 ) ( 1 R₁ - 1 R₂ ) Given _l = 1.6 , _m = 1.33 4 3 , R₁ = +20 cm , and R₂ = +30 cm . Substituting the values: 1 f = ( 1.6 4/3 - 1 ) ( 1 20 - 1 30 ) 1 f = ( 4.8 4 - 1 ) ( 3 - 2 60 ) 1 f = (1.2 - 1) 1 60 1 f = 0.2 1 60 = 1 5 1 60 1 f = 1 300 f = 300 cm