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Due to the inevitability of spherical aberration, there is a practical, but not a fundamental, limit to the resolving power of the electron microscope." [ 1 ] The resolution limit provided by Scherzer's theorem can be overcome by breaking one of the above-mentioned three conditions.
Reproduction of an early electron microscope constructed by Ernst Ruska in the 1930s. Many developments laid the groundwork of the electron optics used in microscopes. [2] One significant step was the work of Hertz in 1883 [3] who made a cathode-ray tube with electrostatic and magnetic deflection, demonstrating manipulation of the direction of an electron beam.
Scherzer's theorem is a theorem in the field of electron microscopy. It states that there is a limit of resolution for electronic lenses because of unavoidable aberrations. German physicist Otto Scherzer found in 1936 [1] that the electromagnetic lenses, which are used in electron microscopes to focus the electron beam, entail unavoidable ...
Developments in ultraviolet (UV) microscopes, led by Köhler and Rohr, increased resolving power by a factor of two. [3] However this required expensive quartz optics, due to the absorption of UV by glass. It was believed that obtaining an image with sub-micrometre information was not possible due to this wavelength constraint. [4]
Above the sample, the electron wave can be approximated as a plane wave. As the electron wave, or wavefunction, passes through the sample, both the phase and the amplitude of the electron beam is altered. The resultant scattered and transmitted electron beam is then focused by an objective lens, and imaged by a detector in the image plane.
In microscopy, NA generally refers to object-space numerical aperture unless otherwise noted. In microscopy, NA is important because it indicates the resolving power of a lens. The size of the finest detail that can be resolved (the resolution) is proportional to λ / 2NA , where λ is the wavelength of the light. A lens with a larger ...
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For example, an electron at an energy of 10 keV has a wavelength of 0.01 nm, allowing the electron microscope (SEM or TEM) to achieve high resolution images. Other massive particles such as helium, neon, and gallium ions have been used to produce images at resolutions beyond what can be attained with visible light.
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