Question

two lenses convex lens of focal length 20cm and concave lens of focal length 5 cm are held close to each other. focal length of the combination of lenses is?​

Answer 1

d= F1 - F2

by using the formula

1÷ F1 + 1÷ F1 - d÷ F1 F2 = 1÷f

for the amergent beam to parallel

P =0, f= a

1÷20-1÷5+ d÷100 = 0

D= 15