Total Internal Reflection: an Optical Fiber Made of Glass with an Index

Total internal reflection: An optical fiber made of glass with an index of refraction 1.50 is coated with a plastic with index of refraction 1.30. What is the critical angle of this fiber at the glass-plastic interface?

A) 90.0°
B) 41.8°
C) 60.1°
D) 50.2°
E) 61.1°