Question

Focal length of 1st lens -5cm and+10 cm what is the focal length of combinatin and power

Answer 1

given,

f1 = -5 cm = - 0.05m and f2 = +10 cm = +0.1 m

let F be the focal length of the equivalent lens and power for same be P .

therefore,

P1 = 1/f1 = 1/ - 0.05= -20 m^-1

P1 = 1/f2 = 1/+ 0.1 = +10 m^-1

hence,

P = P1 + P2 = -20 +10 = -10 m ^-1

and

1/F = 1/f1 + 1/f2

F = f1.f2/ f1 + f2

F = -50/5 = -10cm