You have three questions listed here. Enotes' policy is that each question gets posted separately. I'll answer part 3.
A light microscope uses visible light to illuminate whatever object or specimen that an observer has placed on the microscope stage. The light source could be sunlight or ambient room light. If that is the case, the microscope will use a mirror that the user angles to specific orientations to best light the object. Other light microscopes will use their own light bulb under the stage that shines up. A diaphragm will be used to adjust the amount of light.
An electron microscope uses a beam of electrons to magnify and resolve an image of a specimen. Electron microscopes have much higher magnification powers than light microscopes as well as better resolving power. A light microscope is capable of viewing something as small as bacteria, but not much smaller. Conversely, an electron microscope can be used to view things on the nano scale. Atoms for example. Of course all of that magnification power comes with a much higher price than a typical light optical microscope.