Question

For a convex lens object placed at a distance 7.5cm,its image is found at 10cm from lens.The focal length of lens will be
a) 10cm (b) 20cm
c) 30cm (d) 40cm​

Answer 1

Answer: 30 CM

v= -10cm

f= ?

u= - 7.5 cm

1/v -1/u = 1/f

1/-10 - 1/-7.5 = 1/ f

1/ -10 + 1/ 7.5 = 1/ f

1/-10 + 10/ 75 = 1/f

-15 + 20/150 = 1/f

5 /150 = 1/ f

f = 150 / 5

f= 30 cm

Explanation: