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When an object (e.g., a mineral) is illuminated by a light source (e.g., the sun), some of the light is reflected at the surface and some is refracted into the mineral. The degree of refraction can be quantified by a material-dependent constant of the two media (here, air and mineral), the so-called "refractive index (n)." The higher the refractive index, the slower the light travels and the wavelengths are compressed as they pass through. Generally, minerals with higher density have a higher refractive index because they contain a greater number of atoms and molecules per unit volume; however, there are numerous exceptions (see, for example, Figure 7). The relationship between the refractive indices of the substances ( n₁ , n₂ ) and the angles of incidence and reflection ( θ₉ , θ₀ ) is described by Snell's law of refraction:
Figure 7: The effects of different refractive indices of the two minerals
Cassiterite ( SnO₂ ) and rutile ( TiO₂ ) refract light at specific wavelengths. Although cassiterite is the denser mineral, it has a lower refractive index.