Fundamentals of Microbiology

Principle of Compound Microscope

 Magnification:

            In a compound microscope magnification is obtained by the objective and eyepiece lens system.  The objective lens magnifies the specimen and produces a real image and the eyepiece lens magnifies the real image yielding a virtual image that is seen by the eye.  The total magnification is the product of eyepiece and objective magnification.

Generally microscopes are equipped with three objective lenses

                                                                             Focal length                      Magnification

1.  The low power objective                                    16 mm                                    10 x

2.   The high power objective                                   4 mm                                    40 x

3. Oil immersion objective                                        1. 8 mm                                100 x

Therefore the magnification of a microscope = objective magnification x eye piece magnification

Resolution / Resolving Power:

            It is the ability of the microscope to distinguish between two points which are closely spaced.  The resolving power is the function of wavelength of light and the numerical aperture (NA)

NA is normally given by the manufacturers

For 10x objective it is usually 0.25

       40x objective it is usually 0.65

      100 x objectives it is usually 1.25

Illumination:

            Proper illumination is essential for good magnification and resolution of a microscope.  Artificial light sources are preferred to daylight as the intensity of light, colour and size of beam can be well controlled.

            The light from the source passes into the sub stage condenser, which is then regulated using iris diaphragm and focused into the plane of the object for higher magnification, more light is required and the condenser should be raised and iris diaphragm is fully opened. The condenser does not take part in image formation.