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It consists of two lenses where one lens is compounding the effect of other. The lens close to object is called objective lens (OL) while other lens close to eye is called eye – piece. The objective has a smaller aperture and smaller focal length than those of the eye piece. The object to view is just beyond focal length of objective lens which forms a real, inverted, magnified image on other side. It act as an object for eye piece.

The eyepiece acts as a simple microscope produces further magnification.

CASE 1: When final image is formed at least distance (i.e near point).

Here eye piece is adjusted in such a way that image of an objective lens is taken within its first focal plane and formed final inverted magnified image at near point i.e 25 cm.


\begin{array}{l} m\,\, = \,\,{m_0}\,\, \times \,\,{m_e}\\ \,\,\,\,\, = \,\frac{{{{\rm{v}}_0}}}{{{u_0}}}\left[ {1\, + \,\frac{D}{{{f_e}}}} \right] \end{array}

Since \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{u_0}\,\, \simeq \,\,{f_0}

\begin{array}{l} \,{{\rm{v}}_0}\,\, \simeq \,\,L\\ \therefore \,\,\,\,\,\,\,\,\,\,\,\,\,\,m\,\,\, \simeq \,\,\frac{L}{{{f_0}}}\,\left[ {1\, + \,\frac{D}{{{f_e}}}} \right] \end{array}

Length of the tube:

L\,\, = \,\,{{\rm{v}}_0}\,\, + \,\,\left| {{u_e}} \right|

CASE 2: When final image is at infinity.

Here first inverted image of objective lens is taken at first focal plane of eyepiece forming its final image at infinity. Here magnification is least but least strain on eye to focus the final image.

\begin{array}{l} m\,\, = \,\,{m_0}\,\, \times \,\,{m_e}\\ m\,\, = \,\,\frac{{{{\rm{v}}_0}}}{{{u_0}}}\,\, \times \,\,\frac{D}{{{f_e}}} \simeq \frac{L}{{{f_o}}} \times \frac{D}{{{f_e}}} \end{array}

Thus, compound microscope will have large magnifying power, if both the objective and eyepiece are of short focal lengths.

Note: Both the objective and eyepiece lenses are made up of multicomponent lenses, to minimize various aberrations.