Question

Two thin lens are in contact and the focal length of the combination is 80cm if the focal length of one lens is 20 cm then what would be the power of other lens

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