Here’s some helpful animations and other resources.

By Lucas V. Barbosa – Own work College Help, Public Domain

By Pi-unrolled: John Reid2 pi-unrolled: Ufo karadagli – Pi-unrolled-720.gif, CC BY-SA 3.0

By Inductiveload – Own work, Public Domain

By Traced by User:Stannered – en:Image:Radian-common.png, CC BY-SA 3.0

Other sources include

  1. Longer interactive demonstration of a radian (similar to top above). 
  2. Mathsisfun have a radian page with similar animation too