microscopes it is an instrument for viewing very small objects. A simple microscope is a convex lens of short focal length. A compound microscope gives a larger magnification with less aberrations. It has an objective of short focal length and an eye piece * . The objective forms a real inverted and magnified intermediate image. This image is further magnified by the eye piece. The final magnification, M is the product of linear magnification produced by the objective, M₀ and the angular magnification of the eyepiece, M_e.

M = M₀.M_e

Generally M₀ and M_e are separately labeled.

The numerical aperture, NA of microscope given by,

NA = m sin a

where m is the refractive index of the medium between the object and the objective and a is half angle of the cone of light entering objective. The numerical aperture is increased with oil immersion objectives (see fig. m10).