What is Refractive Index?
Refractive Index n is the speed of light in vacuum relative to the speed of light in the considered medium:
n = speed of light in vacuum / speed of light in medium
Example:
Water has a refractive index of 1.33 which means that the light travels 1.33 faster in vacuum than in water.
Snell’s law
When light enters from a medium with a lower refractive index as for example air into a medium with a higher refractive index as for example water it thus changes its speed.This has as a consequence that a beam of light changes its angle when it passes from one medium with a refractive index n1 to another medium with a refractive index n2.
The ratio of the sines of the two angles is equivalent to the opposite ratio of the refractive indices of the two media. This mathematical relationship is known as Snell’s law.
How does a Digital Refractometer work?
In digital refractometers the light (1) travels from a prism (2) with a high refractive index (normally glass or artificial sapphire) into the sample (3). If the angle of incidence exceeds a certain value, the light is reflected at the prism/sample boundary (see ‘total internal reflection’ in the chapter above). The reflected light is detected by a CCD (or CMOS) sensor (4): The lower the refractive index of the sample being measured, the smaller the critical angle and the bigger the illuminated surface of the sensor. The refractive index of the sample can thus by calculated by the refractometer, using the ratio of the length of the illuminated and the length of the dark region on the CCD.
What are Digital Refractometers used for?
The refractive index is a value specific to a material. It is therefore a quick and easy method for materials characterization and to check the purity of liquids.
Often the refractive index is used for concentration determinations in binary mixtures. The most popular concentration measurement by refractive index is the determination of the sugar concentration in water. There are many refractometers which directly display the results in so called Brix degrees: One degree Brix is 1 gram of sucrose in 100 grams of solution and represents the concentration of the solution as percentage by weight (% w/w). Such instruments are mainly popular in the food industry. The BX-1 portable digital Brix Meter from KEM is a very easy to use instrument for this application.
