question_answer

A convex lens is dipped in a liquid whose refractive index is equal to the refractive index of the lens. Then its focal length will:  [AIPMT 2003]

A)              become small, but non-zero                    

B)       remain unchanged         

C)              become zero     

D)              become infinite

Correct Answer: D

Solution :

From lens maker's formula

(i)

When convex lens is dipped in a liquid of refractive index  then its focal length

or                      ....(ii)

Dividing Eq. (i) by Eq. (ii), we get

....(iii)

But it is given that refractive index of lens is equal to refractive index of liquid i.e., .

Hence, Eq. (iii) gives,

(infinity)