Question

Elaborate on the basic principles of light microscopy and the electron microscopy as to how they work to magnify specimen.

Answer 1

Basic principle of light microscope- the light microscope uses UV/visible light as a source of illumination. The light passes through the specimen kept on the slide. Lenses are used to bend the light and magnify the image of the sample. The objective lens functions to provide a resolved, clear and magnified image. Resolution of a lens is very important, since, it allows to visualize two closely placed small objects as separate entities. A lens with largest possible numerical aperture and light of the shortest wavelength, produces greatest resolution. The refractive index of the medium in which the lens is working governs the numerical aperture of the lens. Refractive index and hence the numerical aperture is increased by replacing air with immersion oil, and thus, a better resolution is achieved (in case of immersion oil, more light enters the objective lens). The light then passes through the eyepiece lens and finally a magnified and resolved image of the sample is viewed.

Electron microscope- The electron microscope uses fast moving, narrow beam of electrons as the source of illumination. It is operated in vacuum. As electrons are generated using an electron gun, it is focused on specimen. The collision of the electrons and the sample results in electron scattering. Magnification is obtained using electromagnetic fields (instead of lenses). The scattered electrons are used to form a magnified image of the sample. The image is obtained on a screen.