Zircon has an index of refraction of 1.923. Taking the speed of light in a vacuum as 3.00 x 10^8 m/s, what is the speed of light inside zircon?

1. The index of refraction is defined by n = c divided by v, so rearranging gives v = c divided by n. 2. Substituting with units, v = (3.00 x 10^8 m/s) divided by 1.923. 3. Carrying out the division gives v = 1.56 x 10^8 m/s. 4. This is a little more than half the vacuum speed, which is a sensible answer for a dense transparent solid. 5. Multiplying by 1.923 instead of dividing gives 5.77 x 10^8 m/s, a speed greater than c and therefore impossible. _Source: OpenStax College Physics (CC BY 4.0), Ch 25 "Geometric Optics", section 25.3 The Law of Refraction_