The compound microscope

The compound microscope uses several lenses to produce a highly magnified image of an object. The diagram shows the microscope in normal adjustment, that is, with the final image at the near point (25 cm from the eye) (distance D from the eye lens). This setting gives the maximum angular size of image without eye strain.

The objective lens produces a real, inverted image of the object 0 at I₁ This is then viewed by the eye lens and this gives a final virtual image at I.

where f_e is the focal length of the eye lens, f_o that of the objective lens and v the distance of I₁ from the objective lens. The resolving power of a microscope depends on the refractive index of the medium above the slide (n) and the angle subtended by the object at the objective (a)