How does temperature generally affect the refractive index of materials?

The refractive index of materials is often influenced by temperature, and generally, it decreases with increasing temperature. As the temperature rises, the density of most materials decreases due to thermal expansion. Since the refractive index is related to how light interacts with the material, a lower density typically results in a lower refractive index.

This phenomenon can be explained by considering how light travels through a medium. When the temperature of a material increases, the molecules move more rapidly and are spaced further apart, which can lead to a reduction in the effective index that light experiences as it passes through. Therefore, as temperature increases, the refractive index commonly decreases, aligning with the correct answer.

The incorrect options reflect a misunderstanding of the relationship between temperature and refractive index. For instance, stating that the refractive index increases with rising temperature overlooks the fundamental effect of thermal expansion on density. Meanwhile, the idea that the refractive index remains constant or fluctuates wildly with temperature changes doesn't align with the common observations and scientific principles governing the behavior of light in different materials.