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Refractometers, devised by Dr. Ernst Abbe, are instruments which can be used to work out the concentration of water based solutions, by measuring the angle of refraction. In the food industry, sugar or salt solutions are measured this way. The refractometer utilises one of two systems, the older scale read transparent system and the newer digital reflection system.

Using Snell’s Law of refraction, the refractive index of a substance can be obtained using the speed of light in a vacuum (v1) and also the speed of light in the substance (. The speed of light in two mediums and the angles of incidence and refraction are both directly proportional to the refractive index. Hence we can work out the refractive index of a substance using the angle of incidence (i) and the angle of reflection (r). Refractive index = = The Transparent System


A sample solution is placed on a prism of much greater refractive index than the sample. If the sample has a low concentration, the angle of refraction is large because there is a big difference in the refractive index between the prism and the sample. On the other hand if the sample has a high concentration, the angle of refraction will be small because it will be similar to that of the prism.

The reading is shown on a scale as a dark shadow line. www. nationalmicroscope. com/faq.html The Reflection System An LED is focused on the underside of the sample carrying prism face, some of this is lost as it is transmitted through the sample being tested but some is reflected back onto an array of photodiodes (a type of detector capable of converting light into a current) creating a shadow line. www. nationalmicroscope. com/faq. html As shown in the diagram light B is both refracted into the solution and then lost and also partly reflected back inside the prism.

Lights A and C, Light A being totally refracted with none being reflected back and light C being totally reflected back into the prism, are both to the right of the reflected ray of light B. As a result a shadow line is formed; which is detected by the photodiodes and can then be converted into a refractive index using: Refractive index = The refractometers internal software can then convert this into a usable scale: oBrix for sugars solutions, oBaume for saline solutions. This shows the concentration percentage of the soluble solids content of the water based solution.

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