# Combination of Lens

- For a system of lenses, the net power, net focal length and magnification are given as follows:
*m*=*m*_{1}*Ã—**m*_{2}*Ã—**m*_{3}*Ã—*â€¦. - In case when two thin lens are in contact: Combination will behave as a lens, which have more power or lesser focal length.
*P*=*P*_{1}+*P*_{2} - If two lens of equal focal length but of opposite nature are in contact, then combination will behave as a plane glass plate and
*F*_{combine}to*n*= âˆž

**Fig. 24**

- When two lenses are placed co-axially at a distance
*d*from each other, then equivalent focal length (*F*) (Fig. 24), and*P*=*P*_{1}+*P*_{2}â€“*dP*_{1}*P*_{2} - Combination of parts of a lens:

**Fig. 25**