Fused Quartz Has an Index of Refraction of 1

Multiple Choice

Fused quartz has an index of refraction of 1.50 for light from a 678-nm source.What is the speed of light for this wavelength within the quartz? (c = 3.00 × 10⁸ m/s)

A) 1.50E+8 m/s
B) 2.00E+8 m/s
C) 2.45E+8 m/s
D) 1.33E+8 m/s
E) 1.50E+8 m/s.