Question

Find the focal length of u=40 v=24
F=?

Answer 1

Answer: f=-15 cm

Explanation:

For concave lens u =24-14(distance of lens)=+10 cm

v=+30

for lens formula, 1/f=1/v-1/u=1/30-1/10

                                        f=-15 cm  

Answer 2

Answer:

1/15

Explanation:

1/F=1/v+1/u

here,

F=24;U=40

1/F=1/24+1/40

LCM Of 24;40=120

In 24 table we get 120 at 5

24×5=120

40×3=120

5+3/120

8/120=1/15

Answer:-1/15