question_answer

Two identical thin plano-convex glass lenses (refractive index 1.5) each having radius of curvature of 20 cm are placed wait their convex surfaces in contact at the centre. The intervening space is filled with oil of refractive index 1.7 The focal length of the combination is     [NEET  2015]

A)  \[-20\,cm\]       

B)       \[-25\,cm\]

C)  \[-50cm\]         

D)       \[50\,cm\]

Correct Answer: C

Solution :

Given

From Lens Maker's formula for the piano convex lens

Here,

and for plane surface

When the intervening medium is filled with oil, then focal length of the concave lens formed by the oil

Here, we have two concave surfaces

So.