Refractive Bending of Light due to Thermal Gradients in Air

The principal feature of an alignment system is a straight line reference. Ideally, light rays can be considered as straight lines. This is however not true in air, where the refractive index is sensitive to both temperature and pressure variations. The required precision of the various alignment devices used for the alignment system of the ATLAS muon spectrometer is at a level where refractive bending becomes non-negligible. A simple setup to measure these refractive effects is presented, and the results are compared to calculations.