Which of the following best describes the relationship between temperature and the density of a material affecting its refractive index?

The relationship between temperature and the density of a material is closely linked to the principles of thermal expansion. As temperature increases, particles within a material gain kinetic energy and move more vigorously, which generally causes them to spread apart. This spreading out of particles results in a decrease in density, as the same mass of material occupies a larger volume.

This decrease in density directly affects the refractive index of the material. The refractive index is defined as the ratio of the speed of light in a vacuum to the speed of light in the material. Since denser materials usually have a higher refractive index, when the temperature increases and density decreases, the refractive index typically decreases as well.

Therefore, as temperature rises, and the material's density decreases, the corresponding effect on the refractive index aligns with the understanding of how thermal properties interact within materials, confirming that the correct choice reflects a fundamental principle in optics and material science.