Refractive index is a property of a substance that expresses how light bends when it passes through that substance, as a result of change in speed. Refractive index for any medium is calculated as:
n = velocity of light in a vacuum / velocity of light in medium
The refractive index is a way to express the optical density of a medium, and it is a useful property for identifying materials of various sorts, as well as for predicting the behavior of light interacting with the substance.
According to wikipedia, the accepted index of refraction for acrylic is 1.490-1.492. According to Cargille labs, which makes liquids with refractive indices that match those of various substances, the exact value for acrylic is 1.4917.
The accepted value is somewhere between 1.490 and 1.492. To be more specific, when the wavelength of light is 587.6 nm, the refractive index is 1.4914.
Hope this helps :)