When light enters a material with a different optical density, it bends. The angle of refraction is the measurement of this new path relative to the perpendicular normal line. Snell's Law uses this angle, along with the refractive indices of the materials, to predict exactly how much a lens or prism will alter the direction of a beam.
Heard on the job
The thick acrylic lens increased the angle of refraction enough to spread the beam across the entire cyclorama.