Problem 10.4. A converging lens of focal length 20 cm is placed in front of a converging lens of focal length 4 cm. What is the distance between the lenses if parallel rays entering the first lens leave the second lens as parallel rays?
Data
Focal length of first lens.f1 =20cm
Focal length of 2nd lens f2 =4cm
To find
Distance between two lens l =?
Solution
Since Rays are parallel therfore the objects p1 and image q2 are at infinity
Distance between two lens is given l=q1+p2......1
Where p2 image distances formed by first lense
1/q1 =1/f1+1/p1=1/20+1/infinity =1/20 +0 _>q1 20cm
Putting these values into equation: l 20+4=24cm
L=24cm answer
