Physics
Photon Interactions with Matter
Quick fact
When a photon is absorbed by an atom, the atom's electron jumps to a higher energy level; when it drops back, a new photon is emitted—this is exactly how neon signs glow.
Why this is interesting
You see a red apple—but why is it red and not blue? The answer lies in how photons, the particles of light, bounce off or get swallowed by the apple's skin.
Read the full explanation
Understanding Photon Interactions with Matter
Photons are tiny packets of light energy. When light hits an object, several things can happen. The photon might be absorbed, meaning its energy is transferred to the material, often exciting an electron. It might be reflected, bouncing off without losing energy, which is why mirrors work. It could be scattered, redirected in different directions, giving us the blue sky. Or it could pass through, as in glass. The specific interaction depends on the photon's energy (its color/frequency) and the material's atomic structure. For example, a red apple absorbs most colors except red, which it reflects, so we see red.
A deeper explanation
The key principle behind all photon-matter interactions is the quantization of energy. Atoms have electrons that can only occupy specific energy levels. A photon can only be absorbed if its energy exactly matches the gap between two electron levels. Similarly, when an excited electron falls back, it emits a photon of that exact energy. This explains why different materials absorb and emit specific colors—their atomic 'fingerprints.' Scattering occurs when a photon's path is altered by interactions with molecules or atoms without permanent absorption, like Rayleigh scattering making the sky blue. Reflection involves photons interacting with the collective oscillations of electrons in a material, often described by classical optics, but at the quantum level it's still a photon interaction. These processes are fundamental to how we see, how solar panels generate electricity (photons knocking electrons loose), and how lasers produce focused beams of light. Understanding photon interactions bridges classical optics and quantum mechanics, revealing the particle nature of light.