A convex lens of power P is immersed in water . How will its power change ?

1) increase

2) remains unchanged

3) decrease

4) increases for red colour and decreases for blue colour .

please ecplain the answer .

thank you .

The relationship between focal length (f), refractive index (n1, n2) and  Radius of curvatures (R1, R2) is given by the lens maker's formula

Now, if the convex lens be placed in air n₁ = 1 and n₂ = 1.5 ,

 and if it be placed in water then n₁ = 1.33, n₂ = 1.5

Put the values you will find after dipping in water the focal length of convex lens increases.

Thus, as power of the lens

P = 1/f

it will decrease. Thus, correct answer is option (3).