34. A plano-convex lens is made of material of refractive index 1.6. The radius of curvature of the curved surface is 60 cm. The focal length of the lens is ?

34. A plano-convex lens is made of material of refractive index 1.6. The radius of curvature of the curved surface is 60 cm. The focal length of the lens is ?

(a) 50 cm

(b) 100 cm

(c) 200 cm

(d) 400 cm

Answer :  (b) 100 cm

\(\frac{1}{f} = (n_{21} - 1) \left( \frac{1}{R_1} - \frac{1}{R_2} \right)\)

\(= (1.6 - 1) \left( \frac{1}{60} - \frac{1}{\infty} \right)\)

\(= 0.6 \left( \frac{1}{60} \right)\)

\(\frac{1}{f} = \frac{1}{100}\)

\( f = 100 \)