Question

A convex lens is placed in water will there any change in focal and give reason

Answer 1

HEY MATE. ...........
YOUR ANSWER IS.....
Yes. This can be understood in the contaxt of lens maker formula.

When convex lens is in air, it’s focal length is given by

1/f(air)=(n-1)[(1/R1)-(1/R2)]……………………(1)

Here, n is refractive index of material of lens relative to air.

R1 and R2 are radii of curvature of two surfaces of convex lens. Surface on the object side has positive radius of curvature and that opposite side has negative radius of curvature.

Now, when the lens is placed in water of refractive index n’ ,the refractive index of the lens is

1/f(water)=[(n-n’)/n’][(1/R1)-(1/R2)]……………………(2)

It is clear from equations (1) and (2) that f(water)>f(air).

Additional Note: If a convex lens is placed in a medium whose refractive index is greater than the refractive index of material of lens, then it will behave as concave lens.

Hope it may help you......
THANKS.......