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