A convex lens having refractive index 1.6 has focal length 12 cm, when it is in air. The focal length of that lens when placed in water is (refractive index of water = 1.28)

Step-by-step solution

Using the Lens Maker's formula for the lens in air: 1 f_a = ( _g - 1) ( 1 R₁ - 1 R₂ ) 1 12 = (1.6 - 1) ( 1 R₁ - 1 R₂ ) 1 12 = 0.6 ( 1 R₁ - 1 R₂ ) Using the Lens Maker's formula for the lens in water: 1 f_w = ( _g _w - 1 ) ( 1 R₁ - 1 R₂ ) 1 f_w = ( 1.6 1.28 - 1 ) ( 1 R₁ - 1 R₂ ) 1 f_w = (1.25 - 1) ( 1 R₁ - 1 R₂ ) 1 f_w = 0.25 ( 1 R₁ - 1 R₂ ) Dividing the two equations: f_w f_a = 0.6 0.25 f_w = 12 2.4 = 28.8 cm f_w = 288 mm Answer: 288 mm