37. 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 ?

37. 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 ?

(a) remain unchanged

(b) become zero

(c) become infinite

(d) become small, but non-zero

Answer :  (c) become infinite

\(\frac{1}{f} = (n_{23} - 1) \frac{2}{R}\)

\(= \left( \frac{n_2}{n_3} - 1 \right) \frac{2}{R}\)

\(= (1 - 1) \frac{2}{R}\)

\(\frac{1}{f} = 0\)

\(f = \infty\)