Abbe refractometer

The Abbe refractometer is an optical device for determining the refractive index of liquids. As Totalreflektometer its measuring principle is based on the fact that the critical angle of total reflection αg depends on an interface on the refractive index of the involved optical media. Here, the total reflection is at the interface of the glass used with known and a large refractive index n ' to be measured and optically thinner liquid with the smaller index of refraction n instead. The optical setup in this developed by Ernst Abbe refractometer allows a precise determination of critical angle with only a thin layer of liquid which is applied between the two glass prisms. About the relationship

Thus be the refractive index n of the liquid charge.

Light refraction is dependent on the wavelength of the incident light. Formerly a sodium vapor lamp was used as a monochromatic light source, usually. Today, usually a light bulb is used in combination with an optical filter. The filter is designed so that it transmits only light of the wavelength 589 nm for the sample. This is the wavelength of the sodium D - line is produced from sodium vapor lamps. This yields the same measurement result with less effort.

Depending on the liquid, the refractive index may be strongly temperature dependent. Therefore, the prisms can be placed during the measuring to a constant, known temperature. The usual test temperature is 20 ° C. It can, however, for various reasons, such as melting, boiling and decomposition temperature of the material being measured, be necessary to measure at a different temperature.

The wavelength and temperature used are indicated as subscript and superscript indices. Here, "D", the indicator for the light of the sodium D line. And the temperature is reported in degrees Celsius.

Example: = Sodium D -line, 20 ° C

Determination of N

First, the liquid is applied to the unfolded double prism (1). Then you look through the right eyepiece, the reticle must be sharp to see. The limit of total reflection is now set to the reticle (2). Now you can read the angle in Ablesemikroskop, here it is 7 ° 46 ' (3).

2

3