Two convex lenses of focal lengths 20 cm and 30 cm are kept at a separation of 15 cm. Then power of the combination is

Equivalent focal length F is give by 1F=1f1+1f2−df1f2=120+130−1520×30am−1⇒ Power P=100FD=10020+10030−15×10020×30D=5.83 D