Fused Quartz Has an Index of Refraction of 1

Multiple Choice

Fused quartz has an index of refraction of 1.56 for light from a 560-nm source.What is the speed of light for this wavelength within the quartz? (c = 3.00 x´ 10⁸ m/s)

A) 1.56 x 10⁸ m/s
B) 1.92 x 10⁸ m/s
C) 2.19 x 10⁸ m/s
D) 4.68 x 10⁸ m/s