What is the primary reason for the change in refractive index with temperature?

The primary reason for the change in refractive index with temperature is due to alterations in the density and molecular structure of materials. As temperature increases, most materials will experience a change in physical state, leading to variations in their density. This change in density affects how light propagates through the material, subsequently influencing the refractive index.

As temperature rises, the kinetic energy of the molecules increases, resulting in greater molecular movement and spacing. This alteration can lead to less dense materials when heated, ultimately causing a reduction in the refractive index, since the light encounters fewer molecules in closer proximity.

In contrast, the other options do not directly account for the temperature dependence of refractive index. Changes in wavelength of light relate more to how light propagates rather than the intrinsic properties of the medium. Variations in the speed of light in a vacuum do not apply since refractive index is fundamentally tied to the properties of the medium, not a vacuum. The influence of external magnetic fields has a negligible effect on the refractive index for most common materials under typical conditions and is more relevant in specialized contexts involving magneto-optic effects.