Two lenses of power $+3.5 D$ and $-2.5 D$ are placed in contact. Find the power and focal length of the lens combination.

(A) When lenses are placed in contact,the effective power $(P)$ of the combination is the algebraic sum of the individual powers of the lenses.
$P = P_1 + P_2$
Given $P_1 = +3.5 D$ and $P_2 = -2.5 D$.
Therefore,$P = +3.5 D + (-2.5 D) = +1.0 D$.
The focal length $(f)$ of the combination is given by the reciprocal of the total power $(f = 1/P)$.
$f = 1 / 1.0 D = 1.0 m$.
Since the power is positive,the combination behaves as a convex lens with a focal length of $1.0 m$.