The Scanning Electron Microscope (SEM) was developed mainly because of the limitations of optical    microscopy. These limitations are caused by two inherent factors involved when using light.
   microscopes: The first is the rather large wavelength of visible light. In theory, an imaging source    should be able to resolve an object the size of half the wavelength of the imaging energy.

        Considering electrons have a much smaller wavelength
        than visible light, the potential for an instrument with
        a much higher resolution exists.

       The next limitation of an optical microscope is its rather
       poor depth of field. The main parameter effecting depth
       of  field is the aperture angle.

          

         The aperture angle is defined as the angle formed between
         a line from the sample through the center of the lens and a
         line from the sample through the edge of the aperture opening.    
         The problem with optical microscopy is that a high power
         objective lens has a short focal length, increasing the aperture
         angle and decreasing the depth of field.

  
         This problem does not exist in a Scanning Electron Microscope
         (SEM). An SEM has a long working distance (the distance
         between the sample and the final lens) and a small aperture
         opening making a very shallow aperture angle and hence a good
         depth of field.

To see examples of an SEM's better resolution and higher depth of field click here

1                                                                                          NEXT