Refractive index of Infrared light through air and perspex
According to this site, the index of refraction of acrylic is as follows: 



Of course the index of refraction of air is 1. 

Edit: By 1, I mean very close to 1 (as it is in a vacuum). If you're interested in the tiny variations from that this NIST site has some equations, but the variations will be much larger in the index of refraction of acrylic, so that will dominate the relative change when the light enters the acrylic from air (or vice versa). Not all acrylics are exactly the same. Wikipedia gives a range of 1.490–1.492 for acrylic, which is around an order of magnitude greater.