The essence of the dispersion phenomenon originates from the refraction of light, and this process follows Snell's Law, that is, when light enters from one medium into another, the ratio of the sine of the incident angle to the refracted angle is equal to the ratio of the refractive index of the two media. Light of different frequencies propagates at different speeds in the same medium, resulting in differences in their refractive index. When polychromatic light (such as white light) enters a prism, this difference in refractive index causes light of different frequencies to refract at different angles, thereby decomposing the polychromatic light into monochromatic light to form a spectrum. For example, in the visible light range, violet light has the highest frequency, the highest refractive index in the medium, and the largest refractive angle; red light has the lowest frequency, the lowest refractive index, and the smallest refractive angle, which is the basic principle of dispersion.
The unique structure of the equilateral prism provides ideal conditions for dispersion. Its three internal angles are all 60 degrees. When a beam of polychromatic light enters from one side of the prism, the light will be refracted twice at two interfaces. The light first enters the prism at the first interface. Since the refractive index of the prism material is greater than that of air, the light is deflected in the normal direction. Then, the light exits the prism at the second interface and is refracted again, and the light deviates from the normal direction. This double refraction process allows light of different frequencies to have a larger angular separation after passing through the prism, thereby significantly enhancing the dispersion effect. Compared with prisms of other shapes, the symmetry and specific angle design of the equilateral prism can disperse light more evenly and effectively.
The optical properties of the prism material are the key factors in determining the dispersion effect. Common equilateral prism materials include optical glass, quartz, etc. Different materials have different dispersion characteristics. Optical glass has good dispersion ability in the visible light band, and its refractive index changes significantly with the frequency of light, which can decompose visible light into a clear seven-color spectrum. Quartz material not only performs well in the visible light band, but also has good transmittance and dispersion performance in the ultraviolet and infrared bands, which is suitable for a wider range of spectral analysis. The dispersion rate of a material (the inverse of the Abbe number) directly reflects its ability to separate light of different frequencies. The higher the dispersion rate, the greater the difference in the refraction angle of light of different frequencies at the same incident angle, and the more obvious the spectral dispersion effect.
The incident angle of light entering the equilateral prism has an important influence on the degree of dispersion. When the incident angle is small, the propagation path of the light inside the prism is short, the change in the refraction angle is relatively small, and the dispersion effect is weak; as the incident angle increases, the propagation path of the light inside the prism becomes longer, the angle difference generated by the two refractions increases, and the dispersion effect is enhanced. However, when the incident angle is too large, total reflection may occur, resulting in the inability of the light to be emitted from the other side of the prism, and thus the dispersion cannot be achieved. Therefore, in spectral analysis applications, the incident angle of the light needs to be precisely controlled to achieve the best dispersion effect. Usually, the incident direction of the light is controlled by adjusting the relative position and angle between the light source and the prism, or by using a collimator.
In spectral analysis instruments, the dispersion process of the equilateral prism works in conjunction with other optical elements. The polychromatic light is first collimated into a parallel beam through a slit and then incident on the equilateral prism. After the monochromatic light is dispersed by the prism, it is spread out in space in the order of different frequencies to form a continuous spectrum. In order to record or observe the spectrum, the instrument is usually equipped with a focusing lens to focus the dispersed light onto a detector (such as a photomultiplier tube, a charge-coupled device CCD, etc.), which converts the optical signal into an electrical signal, and then obtains the spectral data through computer processing. In addition, by rotating the prism or changing the incident angle of the light, the spectrum of different wavelength ranges can be scanned to achieve a comprehensive analysis of the spectral characteristics of the sample.
The dispersion principle of equilateral prism plays an irreplaceable role in spectral analysis. By decomposing polychromatic light into a spectrum, scientists and technicians can analyze the absorption, emission or scattering characteristics of the sample for light of different wavelengths, thereby obtaining information such as the composition, structure and physical and chemical properties of the sample. For example, in chemical analysis, by measuring the absorption spectrum of the sample, the chemical elements and compounds contained in the sample can be determined; in astronomy, by analyzing the characteristic spectral lines of the star spectrum, the temperature, chemical composition and motion state of the star can be inferred. The principle of dispersion provides the basis for spectral analysis, making spectral analysis an indispensable analytical method in modern scientific research and industrial detection.
Although equilateral prism is widely used in spectral analysis, its dispersion principle also has certain limitations. For example, the dispersion of the prism is nonlinear, that is, the dispersion angles of light of different wavelengths are not evenly distributed, which brings difficulties to accurate measurement and analysis. In addition, the dispersion ability of the prism is limited by the material and size, and it may not be able to achieve the ideal resolution for spectral analysis in certain specific wavelength ranges. In order to overcome these limitations, modern spectral analysis instruments often use dispersive elements such as gratings in combination with prisms, or improve the accuracy and efficiency of spectral analysis by optimizing prism design and selecting new materials. With the continuous development of optical technology, the application of equilateral prism in spectral analysis will be further improved and expanded.
