Question

Two similar thin equi-convex lenses, of focal length
f each, are kept coaxially in contact with each
other such that the focal length of the combination
is F. When the space between the two lenses is
filled with glycerine (which has the same refractive
index (u = 1.5) as that of glass) then the
equivalent focal length is F. The ratio F, :F, will​

Answer 1

Answer:

Equivalent focal length in air,

F11=f1+f1=f2 

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

⟹F21=f1+f1−f1

=f1

⟹F2F1=21