Obtain the equation of power for combination of lenses.

Explanation

As power of lens is `P = 1/f`, equivalent power of combination,
`P = P_1+ P_2 + P_3 + ...`
Note that it is an algebraic sum of individual powers.
So sum of the terms on the right side may be positive for convex lenses and negative for concave lenses.
Combination of lenses helps to obtain diverging or converging lenses of desired magnification. It also enhances sharpness of the image.
Combination of lenses is used in camera, microscope, telescope and other optical instruments.