Question

give the expression for the focal length of combination for two lenses separated by distance d​

Answer 1

Explanation:

the lenses of focal lengths f1 and f2are “thin”, the combined focal length f of the lenses is given by 1f=1f1+1f2 1 f = 1 f 1 + 1 f 2 while if the lenses are separated by some distance d then the combined focal length is given by 1f=1f1+1f2−df1f2 1 f = 1 f 1 + 1 f 2 − d f 1 f 2 .