Question

A convexo-concave lens has faces of radii 3 and 4 cm respectively, and is made of glass of refractive index 1.6, its focal length is

• A. 40 cm
• B. -40 cm
• C. 20 cm
• D. -20 cm

Answer 1

Answer: (D)

-20 cm

Answer 2

$\frac{1}{f}=(n-1) \left[\frac{1}{R_1}-\frac{1}{R_2}\right]=(1.6 -2 ) \left[ \frac{1}{-3}-\frac{1}{4}\right]$ Both $R_1\, and \, R_2$ have same sign.