who are using the scanning electron microscope and what is the phycics theory behind this invention?

Answer 1

SEM's are used usually to view very small objects which are beyond the range of optical microscopes. It takes advantage of the fact that electrons can have a wavelength associated with them. They are used by anyone who might use a regular microscope from biological people all the way to integrated circuit manufacture. The disadvantage is that the sample needs to be in a vacuum. Here's a write up ...

http://en.wikipedia.org/wiki/Scanning_electron_microscope

Answer 2

these can be used for many applications. they are useful since they allow the observation of an object up to atomic resolution, this would never be possible with an optical microscope as once you attempt to view something which is smaller than the wavelength of the light you are using diffraction will occur. i have seen them used for investigating the electrical properties of molecules and for observing biological functions (such as forming a scab) on an atomic level.