Choose the correct option regarding lenses in contact.

Options

A. When two lenses are placed in contact, the equivalent focal length is `(1)/(f)=(1)/(f_1)+(1)/(f_2)`
B. When two lenses are placed at a separation `d`, if object is at `infty`, the two lenses can be replaced by a single lens of focal length `(1)/(f)=(1)/(f_1)+(1)/(f_2)-(d)/(f_1f_2)`
C. The equivalent lens is to be placed at a distance `(df)/(f_1)` behind second lens
D. all options are correct.

Answer

A