Two lenses kept together form image of an object placed at a distance of 15cm from lenses at a distance of 80cm.. If focal length of one lens is 20cm., what is the focal length of other lens?

1 Answer
Mar 2, 2017

Focal length of the lens L_2 is 34.286 cm.

Explanation:

When we use two lenses of focal length f_1 and f_2 in conjunction, combined focal length f is given by 1/f=1/(f_1)+1/(f_2). Further, if f is the focal length of the lens, u is the distance of object and v is the distance of image from lenses, the relation between them is 1/f=1/u+1/v

As the object is at a distance of 15 cm and image is at a distance of 80 cm, we have

1/15+1/80=1/f,

but 1/f=1/(f_1)+1/(f_2) i.e. 1/f=1/20+1/(f_2), as focal length of first lens is 20

and hence 1/15+1/80=1/20+1/(f_2) and

1/(f_2)=1/15+1/80-1/20=(16+3-12)/2407/240

i.e. f=240/7=34.286 cm.