Question

The principal axis of convex lens is along x axis.The coordinates of pt. object and its images are (-20;2)and (25;-4) respectively.The focal length of lens will be

Answer 1

The lens formula is as follows :

1/f = 1/u + 1/v

f = focal length

u = Image distance

v = Object distance

Now :

u = 25cm

v = -20 cm

We want f:

1/f = 1/25 - 1/20

1/f = -1/100

Focal length = -100cm

Answer 2

"The convex lens will have the value of an image distance – v, object distance – u and the focal length – f.

Now, the main expression that is used in the calculation will be,

1/f = 1/v – 1/u

As per the question, u = - 20 cm, v = 25 cm, then,

1/f = 1/25 – 1/(-20)

F = 10cm

"