Question

Two identical glass (μg

= 3/2) equi convex lenses of focal
length f are kept in contact. The space between the two
lenses is filled with water (μw

= 4/3). The focal length of the

combination is -

Answer 1

Answer:

Water in between the lens can be considered to be a concave lens of refractive index of water as shown in the figure. This suggests that the radius of curvature of all the lenses is same.

Consider the convex lens-

f

1

=

R

(μ

g

−1)2

=

R

(

2

3

−1)2

hence R=f

For the concave lens,

Power=

f

1

1

=(μ

water

−1)(

R

−2

)=

3f

−2

Power of the lenses get added,

f

combination

1

=

f

1

+

3f

−2

+

f

1

Hence f

combination

=

4

3f

solution