question_answer

Two thin lenses of powers\[~+\mathbf{2D}\] and \[+\text{ }\mathbf{5}\text{ }\mathbf{D}\] are placed in contact. Will the focal length of the combination increase, or decrease?

Answer:

                Power of combination, \[P={{P}_{1}}+{{P}_{2}}=+2+5=+7D\] As power increases, so focal length of the combination decreases.