The principles and usage of common refractometers
An instrument for measuring the refractive index ND and average dispersion NF-NC of transparent, translucent liquids or solids. The instrument is connected with a thermostat and can measure the refractive index ND within the temperature range of 0°C to 70°C, and can also measure the percentage of sugar concentration in the sugar solution. Therefore, this kind of instrument is one of the indispensable common equipment in factories, teaching and scientific research units such as petroleum industry, oil industry, pharmaceutical industry, paint industry, food industry, daily chemical industry, sugar industry and geological survey.
Theoretical basis According to the law of refraction, there is the following relationship between the angle of incidence i and the angle of refraction r: When light enters medium 2 from medium 1, then
=n1,2 (5-3)
In the formula, n1, n2, v1, v2 are the refractive index of the two media 1 and 2 respectively and the propagation speed of light in them. n1, 2 are the pair of media 2.
relative refractive index to medium 1. The refractive index is a characteristic constant of a substance. It is a fixed value for light of a certain wavelength under a certain temperature and pressure.
It can be seen from formula (5-3) that when n2>n1, the refraction angle r is always smaller than the incident angle i. When the incident angle i increases to , the refraction angle increases correspondingly to the maximum value rc, which is called the critical angle. At this time, light passes through the area from OY to OA in medium 2, while the area between OA and OX is a dark area, as shown in Figure 5-3. When the incident angle is , the above formula can be written as:
n2=n1·sinrc (5-4)
That is, when a medium is fixed, the critical refraction angle rc has a simple functional relationship with the refractive index.
use
(1) Open prisms 5 and 6, clean the mirror surface with lens cleaning paper, drop a small amount of the liquid to be measured on the mirror surface, let it cover the entire mirror surface, and close the prism.
(2) Adjust the reflector 7 to maximize the intensity of the incident light, then rotate the prism to make the eyepiece appear half-bright and half-dark, with the dividing line at the intersection of the crosshairs. At this time, the refractive index of the liquid can be read on the ruler through the magnifying glass 2 .
(3) If colored light bands appear, adjust the achromatic compensator so that the colored light bands disappear and the dark interface becomes clear.
(4) After the measurement, open the prism and clean the mirror surface with acetone. You can also use a suction ball to dry the mirror surface. After the experiment, in addition to cleaning the mirror surface, you need to clamp two layers of lens cleaning paper to tighten the two prisms. Close the screws to prevent damage to the mirror.
Correction of ruler zero point
Before use, the refractometer must be calibrated to the zero point of the scale. You can use a standard liquid with a known refractive index (such as pure water = 1.3325), or you can use a "glass block" with a known refractive index attached to each refractometer. Correction. Use a-bromonaphthalene to stick the light side of the "glass block" to the refractive prism 5. Do not close the prism 6. Open the small window behind the prism to let the light in. Use the above method to measure. If the measured value is equal to The refractive index of this "glass block" is different. Turn the correction screw K on the lens barrel to adjust.
