21. Light is travelling through a glass block (n glass = 1.5) towards a clear plastic above the glass. When the light reaches the plastic at an angle of incidence

of 60°from the normal, it reflects entirely back into the glass; no light enters the plastic.What is the smallest possible value for the plastic's index of refraction? A) 1.5 В) 1.3 C) 1.2 D) 1.4 E) 1.0