From the data of four lenses given in `Q.4` which one will you select as objective of a compound microscope and which one as eye lens ? How can the magnifying power of such a microscope be increased ?

Explanation

The objective of a compound microscope should have small focal length and small aperture. Therefore, lens `D` serves as the objective lens. For larger magnifying power, focal length of eye lens should be small (through greater than the focal length of objective lens). Therefore, lens `C` is selected as eye lens.
Magnifying power, `m = (L)/(f_0)(1 + (d)/(f_e))`
`m = (L)/(5) (1 + (25)/(10)) = (7 L)/(10)`
Clearly, `m` can be increased by increasing the length `L` of the microscope tube.