question_answer

The focal length of a piano convex lens is f and its refractive index is 1.5. It is kept over a plane glass plate with its curved surface touching the glass plate. The gap between the lens and the glass plate is filled by a liquid. As a result, the effective focal length of the combination becomes 2f. Then the refractive index of the liquid is

A)  1.5                              

B)        2

C)  1.25                

D)        1.33

Correct Answer: C

Solution :

According to lens maker?s formula The focal length of plane convex lens is or                                                ... (i) The focal length of liquid lens is      \[\frac{1}{{{f}_{1}}}=-\frac{2({{n}_{1}}-1)}{f}\]                      [Using (i)] Effective focal length of the combination is