Question

a concave lens of focal length 25 cm and a convex lens of focal length 20 CM are placed in contact with each other what is the power of the combination​

Answer 1

Answer:

Given that - F1 = - 25 cm

F2 = + 20 cm

Combined focal length , 1 / F = 1 / F1 + 1 / F2

1 / F = - 1 / 25 + 1 / 20

1 / F = ( -4 + 5 ) / 100

1 / F = 1 /100

F = 100cm = 1 m

as per by formulae ,

P = 1 / F ( In Metre) = 1 / 1 =1 Dioptre= 1D