Question

A convex lens made of glass is immersed in water.As a result,its focal length will be?

Answer 1

we know the formula for finding focal length,
      $\bold{\frac{1}{f}=(\mu-1)[\frac{1}{R_1}-\frac{1}{R_2}]}$
where,  $\bold{\mu=\frac{\mu_{lens}}{\mu_{medium}}}$
here you can see that,
   focal length depends upon refractive index of lens with respect to medium.
refractive index of lens = 1.5 
refractive index of water = 1.3
so, refractive index of lens with respect to water = 1.5/1.3 = 1.15 

now, Let initial focal length of lens is f in air 
1/f = (1.5 - 1)[1/R₁ - R₂]-----------------(1)
then, focal length will be ,after immersing the lens into water is  f' 
1/f' = (1.15 -1)[1/R₁ - 1/R₂ ]---------------------(2)

from equations (1) and (2) it is clear that 
 1/f  > 1/f'
⇒ f < f'

hence, after immersing of lens in water focal length of lens increases.

  

Answer 2

A convex lens made of glass is immersed in water.As a result,its focal length will  increase.

Experiment To prove :

1. Consider cylindrical vessel like glass tumbler

2. Its height must be greater than the focal length of lens.

3. Keep a  black stone inside the vessel at its bottom.

4. pour the water into the vessel such that the height of water level from the top of stone is greater than focal length of lens.

5. Now dip the lens horizontally using the circular lens holder.

6. set the distance between stone and lens that is equal to or less than focal length of lens.

7. Now see the stone through lens.

8. if we dip the lens upto a height greater than its focal length of air still we can see the image.

9. This shows that the focal length of convex lens has increased in water.