Question

we are given a convex lens of focal length 20cm draw a ray diagram to show the nature size and position of the image formed when the object is kept at the distance of 55 CM

Answer 1

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?

Answer

Focal length of the lens L2 is 34.286 cm.
When we use two lenses of focal length f1 and f2 in conjunction, combined focal length f is given by 1f=1f1+1f2. 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 1f=1u+1v

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

115+180=1f,

but 1f=1f1+1f2 i.e. 1f=120+1f2, as focal length of first lens is 20

and hence 115+180=120+1f2and

1f2=115+180−120=16+3−122407/240

i.e. f=2407=34.286 cm.