When a straw is placed into a glass of water, the straw appears bent. Now if a straw is placed in a glass with water containing dissolved sugar, the straw should appear even more bent (see illustrations). This phenomenon is known as the principle of light refraction. Refractometers are measuring instruments which put this phenomenon of light refraction to practical use. They are based on the principle that as the density of a substance increases (e.g. when sugar is dissolved in water), its refractive index (how much the straw appears bent) rises proportionately. Refractometers were devised by Dr. Ernst Abbe, a German/Austrian scientist in the early 20th century.
The prism in refractometers has a greater refractive index than the sample solution. Measurements are read at the point where the prism and solution meet. With a low concentration solution, the refractive index of the prism is much greater than that of the sample, causing a large refraction angle and a low reading. The reverse (lower refraction angle and higher reading) would happen with a highly concentrated solution.
There are two detection systems for refractive index: transparent systems and reflection systems. Hand-held refractometers and Abbe refractometers use transparent systems, while digital refractometers use reflection systems.
Transparent Systems
The detection system for hand-held refractometers (transparent system) is summarized below.
1. In the figure below the detection is done by utilizing the refractive phenomenon produced on the boundary of the prism and sample. The refractive index of the prism is much larger than that of the sample
2. If the sample is thin, the angle of refraction is large (see "a") because of the large difference in refractive index between the prism and the sample.
3. If the sample is thick, the angle of refraction is small (see "b") because of the small difference in refractive index between the prism and the sample.
Reflection Systems
In the figure below, Light A, being incident from the lower left of the prism, is not reflected back by the boundary, but exits through the sample. Light B is reflected by the boundary face to the right, directly along the prism boundary. Light C, having an incident angle too large to be let through to the sample side, is totally reflected toward the lower right of the prism.
As a result, a boundary line is produced dividing light and dark fields on either side of the dotted line "B' " in the figure. Since the angle of reflection of this boundary line is proportional to refractive index, the position of the boundary line between light and dark fields is caught by a sensor and converted into refractive index.
In the figure below, Light A, being incident from the lower left of the prism, is not reflected back by the boundary, but exits through the sample. Light B is reflected by the boundary face to the right, directly along the prism boundary. Light C, having an incident angle too large to be let through to the sample side, is totally reflected toward the lower right of the prism.
As a result, a boundary line is produced dividing light and dark fields on either side of the dotted line "B' " in the figure. Since the angle of reflection of this boundary line is proportional to refractive index, the position of the boundary line between light and dark fields is caught by a sensor and converted into refractive index.
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