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Interferometry is a method of measurement related to physical quantities based on the interference law of coherent waves. Usually, mechanical waves (such as sound waves), light waves, radio waves, etc. are used for interference. Among these interferometries, light wave interferometry is the one that has been most widely used. It is mainly used to measure the length, angle, wavelength,refractive index of gas or liquid, and detect the quality of various optical elements.
Figure 2.5.1 Schematic diagram of wedge interference
The wedge-shaped interferometry is a simple and common method to measure and detect physical quantities. As shown in Figure 2.5.1, the interference of light in a wedge-shaped film is based on the principle of equal thickness interference of light. On one hand, this method can be used to measure the diameter of filaments (or other objects with small thickness). Firstly, the filament to be measured is placed at one end between two flat glass plates to form a wedge-shaped air gap.Then, when the monochromatic light of wavelength λ is vertically illuminated on the glass plate, a group of equal-spaced interference fringes between bright and dark parallel to the wedge edge are formed on the upper surface of the air gap. Moreover, the difference of air gap thickness between two adjacent bright (or dark) stripes is half a wavelength. Therefore, if the total length from the edge of the wedge to the filament is L and the number of stripes per unit length is n , then the diameter of the filament is:
On the other hand, the wedge-shaped interferometry can also be used to detect the flatness of the optical surface. The surface to be measured and a standard flat optical glass are used to form a wedge. When the monochromatic light is vertically illuminated on the wedge, the equal thickness interference fringes can be observed. If some fringes are bent, the surface to be measured is not flat at the position corresponding to the fringes.
In addition, interferometers for many other applications can also be made according to the in terference principle, such as the Michelson interferometer for precise length measurement, the refractive interferometer for refractive index measurement, the celestial interferometer for celestial body measurement, the industrial micro-interferometer for measuring the roughness of the mechanical parts surface, etc.