ABSTRACT
A microscope can be de ned as an instrument that uses one or several lenses to form an enlarged (magni ed) image. e term microscope comes f rom t he Greek “mikros” = small a nd “skopos” = to look at. Microscopes can be classi ed according to the type of electromagnetic wave employed, depending on whether or not this wave is transmitted through the specimen. In transmission microscopes, the electromagnetic wave passed through the specimen is di erentially refracted and absorbed. e most common t ypes o f t ransmission m icroscopes a re t ransmitting light microscopes, in which v isible spectrum or selected wavelengths pa ss t hrough t he specimen, a nd t ransmission ele ctron microscopes (TEMs, Figure 40.1a) where the source of illumination is a n electron beam.1−6 Electron beams can a lso be passed
over the surface of the specimen, thus causing energy changes in the sample. ese changes are detected and analyzed to give an image of the specimen. is type of microscope is called scanning electron microscope (SEM, Figure 40.1b).2,4,7 e optical paths of the illumination beam in light microscopes and TEMs are nearly identical. Both types of microscopes use a condenser lens to converge the beam onto the sample. e beam penetrates the s ample a nd t he ob jective len s f orms a m agni ed image, which is projected to the viewing plane. SEM is nearly identical to T EM w ith re spect to t he i llumination s ource a nd t he c ondensing o f t he b eam o nto t he s ample. H owever, sig ni cant features distinguish SEM from TEM. Before contacting the sample, the SEM beam is de ected by coils that move the beam in scan p attern. en a nal len s (which i s a lso c alled ob jective lens) condenses the beam to a ne spot on the specimen surface.
