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 centres of the lenses to be coincident. Let this central point is P.
For lens A
Considering equivalent lens of focal length f, then
Purpose of combination of lens.
(a) To meet desired magnification
(b) It also enhance sharpness of the image.
(c) To make 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.
(1) If several thin lenses of focal length are in contact, then effective focal length is
(2) Equivalent power of the combination is an algebraic sum of individual powers.