Magnification is the enlargement of an image, or portion of an image. In a scanning electron microscope this is achieved by scanning a smaller area. In the images, the beam is indicated by arrows on a sample
As a smaller region is scanned, what we see is the object getting bigger. In the micrographs the image is magnified from 900x to 10,000x across the three frames. This is a picture of the tiny spheres that are produced from a lit party sparkler. Magnification should be shown on an image (electron micrograph) as a scale bar with a size measurement of the bar on top or beside it. The bar is used to work out the actual magnification of the image since it scales with the size of the image.
The SEM image is effectively made up of lines of image points, each point being the size of the beam spot at the sample surface. The ability of the SEM to resolve fine structures is limited by the diameter of this spot size (probe size). It is also limited by the number of electrons contained within the probe. If the probe is too small in relation to the area being imaged, it spends too little time on each image point to provide sufficient signal to form a good quality image. There is a finite relationship between magnification and the optimum probe size and it does vary from specimen to specimen.
Each dwell time (seen as a spot in the image) generates electrons that are used to make up the image on the screen. We see edges and dips and bumps on a sample because of changes in the amount of electrons coming off the sample at that point. As we go up in magnification, and drop our beam probe size down to a smaller and smaller spot, we see more detail (see line 2 in the diagram). But there is a limit. The limit of magnification is the point where no variation in signal (electrons generated from the sample) is obtained from adjoining points on the sample. This performance limit is dependent on the composition and structure of the specimen being examined. For example, specimens such as metals with a high atomic number (Z) produce a high yield of electrons and achieve a higher useful magnification than low Z samples (e.g. carbon and plastic).