Question

Two thin lenses are in contact and the focal length of the combination is 80 cm. If the focal length of one lens is 20 cm, then the power of the other lens will be(a) 1.66 D(b) 4.00 D(c) –100 D(d) –3.75 D

Answer 1

P is the power of combination of two given, lens which power are P1 and P2

we know,

power =1/focal length

we know,

P = P1 + P2

1/feq = 1/f1 + 1/f2

1/80 =1/20 + 1/f2

1/f2 = 1/80 -1/20 = -3/80

f = -80/3 cm = -26.666 cm = -0.2666 metres

Calculate power by

1/f(in m)

1/-0.2666

Equals

-3.75D

Hence your answer is 'D'