Unlike the compound light microscope which uses light to image the sample, the electron microscope uses electrons to generate an image of the sample. The SEM has a much greater resolution capability than the light microscope because the wavelength of the electrons is about 100,000 times smaller than the wavelength of light.
Resolution is the ability to distinguish between two closely-spaced points.
The best resolution of the light microscope is 0.2 m or 200 nm. The resolution of our scanning electron microscope is 3 to 6 nm. That's almost 100 times better than the light microscope. This is why we can see so much more detail with electron microscopes than light microscopes. The highest useful magnification of an objective lens in the light microscope is 100 times.