Question

Two identical lenses made of the same material of refractive index 1.5 have the focal length 12 cm. these lenses are kept in contact and immersed in a liquid of refractive index 1.35. the combination behaves as (a)

Answer 1

Answer is :
Combination of lens behaves as Convex lens of focal length 27cm

Explanation :

f1=f2=12cm

When lenses are kept in water :
1/f=1/f1+1f2
1/f=1/12+1/12
=2/12
1/f=1/6
f=6cm

Using lens makers formula:
1/f=(n-1)(1/R1-1/R2)
1/6=(3/2-1)(1/R1-1/R2)
1/R1-1/R2=1/3

Now in liquid:
1/f=(1.5/1.35-1)(1/R1-1/R2)
1/f=0.11x1/3
f=27cm