In the context of optics, which materials generally exhibit a decreasing refractive index with rising temperature?

The correct answer is that both solid and liquid materials can exhibit a decreasing refractive index with rising temperature. This phenomenon is primarily due to the changes in the density and molecular interactions within a material as temperature increases. For many substances, increasing temperature leads to thermal expansion, which can reduce the density of a material and subsequently affect how light interacts with it.

In liquids, the effect is often more pronounced as they tend to have greater thermal expansion compared to solids, leading to a notable decrease in refractive index. For some solids, particularly certain crystalline and amorphous materials, a decrease in refractive index with temperature can also occur, although it may not be as universally applicable as it is with liquids.

Gaseous materials generally show a more complex relationship, and gas refractive indices typically increase with temperature under normal conditions because as temperature increases, there may be increased molecular activity leading to more scattering. Thus, the broad applicability to both solids and liquids makes this the correct choice. Overall, materials respond differently to heat, but the trend of a decreasing refractive index with temperature is observable in both liquid and solid materials in many cases.