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 . 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
Two similar equi-convex lenses of focal length f each are held in contact with each other.
The focal length of the combination is given by
For glycerin in between lenses, there are three lenses, one concave and two convex
Focal length of the curve lens is given
Now, equivalent focal length of the combination is given by