The behavior of light in transparent solids and metals can be explained by the difference in the electronic structure and conductivity of these materials.
In transparent solids, such as glass or diamond, light is not reflected by valence electrons but rather slowed down and refracted as it passes through the material. This is because the valence electrons in these solids are tightly bound to their respective atoms and participate in localized bonding. When light interacts with these materials, it induces oscillations in the electric fields of the atoms, known as polarization. These polarization effects delay the propagation of light, resulting in a slower speed. This phenomenon is described by the refractive index, which measures the degree of bending or slowing down of light in a material compared to its speed in a vacuum.
On the other hand, in metals, light is reflected by conduction electrons. Metals have a unique electronic structure characterized by a "sea" of delocalized electrons. These conduction electrons are not tightly bound to individual atoms but are free to move throughout the metal lattice. When light interacts with a metal, the electric field of the light wave induces the conduction electrons to oscillate collectively. This collective oscillation of electrons, known as plasmons, creates an oscillating electric field that acts as an effective reflector for the incident light. Consequently, metals exhibit high reflectivity for visible light.
The difference in behavior arises from the contrasting electronic properties of transparent solids and metals. In transparent solids, the valence electrons are localized and participate in localized bonding, leading to slower light propagation. In metals, the presence of delocalized conduction electrons allows for strong interactions with the incident light, leading to reflection.