Fizeau Interferometer to Measure Refractive Index of Air With Permanent Vacuum Prism Etalon

Over the years much research has been done into the measurement of the speed of light. The speed of light has had many applications in measurements since it has been determined with the required accuracy. The refractive index of the medium in which the measurements are performed is critical in order to use the speed of light for accurate measurements. In dimensional metrology, most measurements are performed in air, and therefore, the refractive index of the air must be determined to a higher accuracy than the dimensional measurements performed. The most accurate way to determine the refractive index is to use a refractometer. Previous research by the authors looked at using a prism as the vacuum cell instead of a cylinder. The main advantage was that it eliminates the use of a vacuum pump. In this research, a new prism was designed to overcome difficulties with the previously used prism. A new method to use an optical flatness interferometer with a Fizeau interferometer layout to measure the air/vacuum ratio was researched and the results are presented here. The results achieved in the measurement method of the refractive index of air were accurate to 6 × 10 -8 , which compares well with other refractometers with the added described advantage.