SPHERICAL ABERRATION ELECTRON MICROSCOPY (SEM)
Spherical aberration is the most important type of aberration in the objective lens. Cs are always positive for a lens that is rotationally symmetric for the optical axis.
The third-order spherical aberration in TEM has been fixed successfully. The TEM also has spherical aberration because electrons moving around the edge of the lens are bent more than those moving along the axis. So, none of the electrons will reach a single focal point. To get rid of spherical aberration, an aperture is used to get rid of some of the electrons on the edges of the object.
The main difference between SEM and TEM is that SEM makes an image by detecting electrons that have been reflected or knocked off, while TEM makes an image by using electrons that are passing through the sample
The scanning electron microscope (SEM) takes pictures by moving a high-energy beam of electrons back and forth over a sample. When the electrons hit the sample, they make secondary electrons, electrons that bounce back, and X-rays that are unique to the sample.
SEM is often used to look at the microscopic structure and chemical makeup of a wide range of materials. The main parts of the SEM are a source of electrons, electromagnetic lenses to focus the electrons, electron detectors, sample chambers, computers, and displays to see the images.
The SEM is a piece of equipment that uses electrons instead of light to make a very large image. At the top of the microscope is an electron gun, which sends out a beam of electrons. The electron beam moves in a straight line through the vacuum-sealed microscope.