1. Refractometers utilise a prism which possesses a much a greater refractive index than the sample solution to be measured. Measurements are made possible using the refractive phenomena which arise at the interface of the prism and the sample solution.
2. In the case of a weak sample solution, the difference between the refractive index of the solution and that of the prism is great, therefore the angle of refraction is large (see A on the diagram below).
3. In the case of a strong sample solution, the difference between the refractive index of the prism is smaller and therefore the angle of refraction is smaller (see B on the diagram below).