Which statement correctly describes the refractive index (RI) of air and gem materials?

• A. The RI of air is assumed to be 0.5, and all gem materials have RIs greater than air.
• B. The RI of air is assumed to be 1, and all gem materials have RIs greater than 1.
• C. The RI of air is assumed to be 1.5, and gem materials have lower RIs than air.
• D. The RI of air is assumed to be 2, and gem materials range around 1.

Answer: (B)

Refractive index compares how fast light travels in air versus a material, and in gemology we treat air as the reference medium with an RI essentially 1.0. In reality air’s RI is about 1.0003 at standard conditions, but we approximate it as 1.0 for practical calculations. Gem materials, being denser, have refractive indices greater than 1.0 (examples range from roughly 1.4 up to about 2.4, depending on the gemstone). So the statement that air is assumed to have RI of 1 and all gem materials have RIs greater than 1 correctly reflects the standard convention and the basic behavior of light as it enters a gemstone. The other options assign implausible values to air or claim gem materials have lower RIs than air, which contradicts how refraction works in common gemstones.