Abbe refractometer working principle is based on the critical angle. The sample is placed between two prisms - measuring and illuminating. Light enters the sample from the illuminated prism, refracts to a critical angle on the lower surface of the prism to be measured, and then the telescope is used to measure the boundary conditions between the bright and light regions. The telescope respects the image, so the dark field is at the bottom, even though we expect it to be in the upper part of the field of view. Knowing the angle and refractive index of the measuring prism, it is not difficult to calculate the refractive index of the sample.
The surface of the illuminated prism is matted, so that light enters the sample at all possible angles, including those nearly parallel to the surface. If you have seen our page on a critical angle refractometer, you already know that these rays are important for refractometer work.
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