Derive the relation for effective focal length of an optical system made up of two thin lenses in contact.
Answers
Consider two lenses A and B of focal length f1 and f2 placed in contact with each other.
The image of point object O will be at I1 formed by lens A, which act as a virtual object for second lens B producing the final image at I.
Since lenses are thin; therefore we assume the optical centers of the lenses to be coincident.
Let this central point is P.
For lens A
Considering the equivalent lens of focal length f, then
Purpose of a combination of the lens.
(a) To meet the desired magnification
(b) It also enhances the sharpness of the image.
(c) To make the final image erect.
(d) To remove certain defects in the lens.
Such a system of combination of lenses is commonly used in designing lenses for cameras, microscopes, telescopes and other optical instruments.