Question

An object is placed 25 cm in front of the convex lens whose focal length is 10 cm. (a) Find the image distance and where it is located Now a concave lens is placed 6 cm behind the convex lens and a new image is formed 20 cm behind the convex lens. Draw a rough diagram and indicate the length measurements (b) Calculate the focal length of the concave lens (hint: the image formed by the convex lens will be the object to the concave lens)

Answer 1

a)

$d=\frac{fF}{f-F}=16.7~cm,$ behind the lens,

b)

$\frac 1{d-l}+\frac 1{F'}=\frac 1{d'-l},$

$F'=\frac{(d'-l)(d-l)}{d-d'}=44.8~cm .$