The use of the microscope is a subject of general interest in courses on medicine and life sciences. White light illumination, phase-contrast and even interference microscopy are techniques currently used in such courses. An experimental demonstration of the working principles of a phase-contrast microscope for individual students and also for small groups is described, with particular emphasis on the importance of the phase-contrast method in making visible almost totally transparent objects.

https://iopscience.iop.org/article/10.1088/0143-0807/6/3/003/pdf

Phase-contrast microscopy

An optical crystal (a multilayer diffraction plate) which optically simulates electron diffraction by thin crystals has been developed. The lattice was constructed with appropriate lattice constants to allow a direct optical simulation of electron diffraction by mica. The resulting optical-diffraction patterns agree with theory and vividly illustrate Laue zones. Optical-diffraction patterns for different crystal orientations are directly compared to electron-diffraction patterns for the same orientations of a thin mica crystal. The breadths of Laue zones diminish as a crystal becomes thicker, reducing the diffraction pattern to the Laue spots described by Bragg’s law.

Optical simulation of electron diffraction of thin crystals

A simple setup is described that simulates an electron microscope imaging a two-dimensional lattice. The diffraction pattern in the back focal plane is large enough to be easily observed and manipulated. The origin of contrast can be demonstrated by filtering out the image contributions of the individual Fourier components. The experiment can be used to demonstrate Abbe's theory of the microscope, origin of phase contrast, and two-dimensional Fourier transforms.

Dieter G. Ast

Papers

Optical Simulation of the Origin of Contrast in the Electron Microscope