# 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 $$F_1$$. When the space between the two lenses is filled with glycerine (which has the same refractive index ($$\mu = 1.5$$) as that of glass) then the equivalent focal length is $$F_2$$. The ratio $$F_1 : F_2$$ will be :

#### Correct option is B. $$1 : 2$$

Equivalent focal length in air,

$$\dfrac{1}{F_1} = \dfrac{1}{f} + \dfrac{1}{f} = \dfrac{2}{f}$$

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

$$\implies \dfrac{1}{F_2} = \dfrac{1}{f} + \dfrac{1}{f} - \dfrac{1}{f}$$

$$ = \dfrac{1}{f}$$

$$\implies \dfrac{F_1}{F_2} = \dfrac{1}{2}$$