Ernst Abbe was not the first and not the only scientist who developed a convenient refractometer, but  his refractometer is – in several varied and updated versions – the most widespread and most common refractometer nowadays. Although the refractometer have been replaced at the beginning of the 20th century by more specialised spectroscopic methods, it is still an important instrument for the purity control of liquids. Consequently the Abbe refractometer developed from an laboratory instrument in basic research to an indispensable monitoring tool in industry. This experiments utilises an Abbe type refractometer. As light source a green LED is used for best eye sensitivity. Before the invention of LED, the standard yellow line of Sodium has been used. Due to the manifold of available wavelengths of the LED, ranging from UV to IR, many tasks can be performed in industrial manufacturing, chemistry and food industry. When placing drops of a test liquid on top of the prisms the shadow line of the light beam on the screen is shifted. From this value and the device parameter the index of refraction is calculated. Liquids with known different index of refraction are provided.