Two lenses of power -15 D and +5 D are in contact with each other. The focal length of the combination is
- (A)-20 cm
- (B)-10 cm✓ Correct
- (C)+20 cm
- (D)+10 cm
Answer: (B)
Solution
Lenses placed in contact add their powers, so the combined power is dioptre. The focal length is the reciprocal of the power, m, that is cm. The negative sign shows the combination acts as a diverging one, the stronger concave lens prevailing over the weaker convex lens.
JEE Main · 2007 · 29 Apr 2007 · Offline · Single correct option