Question

Two thin convex lens of power +5 and +4 diopter are placed co-axially at distance 10cm .find the focal length of the combination

Answer 1

hope it helps you.......

Answer 2

power of first convex lens, $P_1$ = +5D

so,focal length of first convex lens, $f_1$ = 100/5 = 20cm

similarly, power of 2nd convex lens , $P_2$ = +4D

so, focal length of convex lens, $f_2$ = 100/4 = 25cm

now use formula,

$\frac{1}{f}=\frac{1}{f_1}+\frac{1}{f_2}-\frac{d}{f_1.f_2}$

where d is the distant between convex lens when both are placed co-axially

now, 1/f = 1/20 + 1/25 - 10/(20 × 25)

1/f = (5 + 4)/100 - 2/100

1/f = 9/100 - 2/100 = 7/100

hence, f = 100/7 cm ≈ 14.3 cm