Question

two thin convex lens as made of glass are placed in close contact with each other. the focal length of each lens is f. if the space between the lens is filled with same glass material as shown then the effective focal length of the system is​

Answer 1

Answer:

Equivalent focal length in air,

F

1

1

=

f

1

+

f

1

=

f

2

When glycerin is filled inside, glycerin lens behaves like a diverging lens of focal length (-f)

⟹

F

2

1

=

f

1

+

f

1

−

f

1

=

f

1

⟹

F

2

F

1

=

2

1

Answer 2

Answer:

let focal length of combination be F

then

1/F = 1/2f +1/2f =2/2f =1/f

so F=f