Two similar thin equiconvex lenses, of focal length 
each, are kept coaxially in contact with each other such that the focal length of the combination is 
. When the space between the two lenses is filled with glycerin (which has the same refractive index 
as that of glass) then the equivalent focal length is 
. The ratio 
will be
Text Solution
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: According to lens maker ’ s formula
Two similar equi ‐ convex lenses of focal length 
each are held in contact with each other.
The focal length 
of the combination is given by
(i)
For glycerin in between lenses, there are three lenses, one concave and two convex.
Focal length of the concave lens is given by
Now, equivalent focal length of the combinationis,
Dividing equation (i) by (ii), we get 
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