Analysis on the working principle of Abbe refractometer
Refractometers are designed based on this principle. Figure 5-4 is a schematic diagram of the instrument structure. Its main parts are two right-angle prisms PI and PII. There is a gap of about 0.1 to 0.15mm between the rough surface of prism PI and the optical plane mirror AD of PII, which is used to hold the liquid to be measured and to form a gap between PI and PII. thin layer. After the light enters the prism PI from the reflector, it is diffused because the surface is rough frosted glass, and passes through the measured liquid in the gap from various angles; it enters the prism PII. As we know before, the light entering the prism PII from all directions All light rays are refracted, and their refraction angles fall within the critical angle rc (because the refractive index of the prism is greater than the refractive index of the liquid, all light rays with incident angles from to to can be refracted through the prism). The light with the critical angle rc passes through the prism PII and hits the eyepiece. At this time, if the crosshairs of the eyepiece are adjusted to the appropriate position, you will see half light and half darkness on the eyepiece.
It can be proved from the principle of geometric optics that the relationship between the refractive index n of the liquid in the gap and rc is:
n liquid=sinB
B is a constant for a certain prism, and prism n is also a constant value at a constant temperature. So the refractive index n liquid of a liquid is a function of the angle rc. The refractive index of the liquid can be calculated from rc. The reading rc has been converted into the value of n liquid on the refractometer, and the value of n liquid can be read directly.
Under specified conditions, the refractive index of a liquid varies depending on the wavelength of the monochromatic light used. If ordinary white light is used as the light source, colored light bands will appear at the boundary between light and dark due to dispersion, making the boundary between light and dark unclear. In order to use white light as the light source, two "Amici" prisms each consisting of three prisms are installed in the instrument as compensation prisms (the upper "Amici" prism can be rotated) to adjust their relative positions. , when properly oriented, the dispersed light coming out of the refracting prism below can be turned into white light again, eliminating color bands and making the boundary between light and dark clear. At this time, the refractive index measured with white light is equivalent to the refractive index nD measured with sodium light D line (wavelength 5890 nm).
The refractive index is one of the characteristic constants of matter, and its value is related to temperature, pressure and wavelength of the light source. The symbol refers to the refractive index of the substance when sodium light D line is used as the light source. Temperature has an effect on refractive index. When the temperature of most liquid organic substances increases, the refractive index decreases to , while the relationship between the refractive index of solids and temperature is irregular and generally does not exceed . Usually changes in atmospheric pressure have little effect on the numerical value of the refractive index, so the effect of pressure is only considered in very precise work.
