The Core Mechanism of Light Reflection
Light reflection occurs when electromagnetic waves encounter a boundary between two different media and bounce back, rather than passing through or being absorbed. At a microscopic level, this phenomenon is governed by the interaction between photons (light particles) and the electrons within the atoms of the material. When light strikes a surface, the electrons in the atoms absorb the photon's energy and then almost instantaneously re-emit it as another photon of the same energy and frequency, but in a different direction.
Electron Interactions and Energy Re-emission
The 'cause' of reflection boils down to the electron configuration and energy levels within a material. In opaque materials, especially metals, there are many free electrons. When incoming light's electromagnetic field oscillates, it causes these electrons to oscillate in sync. These oscillating electrons act like tiny antennas, radiating new electromagnetic waves (photons) back into the original medium. Since the electrons are very good at re-emitting this energy almost immediately, little energy is converted to heat or transmitted, resulting in efficient reflection.
Specular vs. Diffuse Reflection
The way light reflects depends on the surface's smoothness. If the surface is microscopically smooth (like a mirror), all incoming parallel light rays reflect in a single, coherent direction, known as specular reflection. If the surface is rough, the light rays hit at various angles at the microscopic level and scatter in many different directions, leading to diffuse reflection, which is how we see most objects that aren't mirrors, as the light reflects off them in all directions.
Why Some Materials Reflect More Than Others
The extent of reflection is determined by the material's optical properties, specifically its refractive index and absorption characteristics. Materials with a high refractive index difference compared to the surrounding medium tend to reflect more light. Metals are excellent reflectors due to their abundance of free electrons that readily re-emit light. Conversely, dark, matte surfaces absorb most incident light, converting it to heat, and reflect very little. This difference is why a black shirt gets hotter in the sun than a white one.