Quantum Electrodynamics (QED) is a fundamental theory in physics that describes the interactions between electrically charged particles and the electromagnetic force. At its core, QED explores the interplay between photons, which are particles that embody the electromagnetic force, and atoms, the building blocks of matter. This essay provides an introduction to the principles of QED, focusing on the interaction between photons and atoms.

When a photon approaches an atom, it can be absorbed by an electron, causing the electron to transition to a higher energy level. This process is known as photon absorption. Conversely, when an excited electron transitions to a lower energy level, it emits a photon, a process known as photon emission.

The interaction between photons and atoms is a crucial aspect of QED. This interaction can be described by the Feynman diagrams, which are graphical representations of the mathematical expressions that describe the behavior of subatomic particles. In QED, the interaction between photons and atoms is mediated by the exchange of photons.

Photons are massless particles that represent the quantum of light or electromagnetic radiation. They have zero rest mass, and their energy (E) is related to their frequency (f) by the equation E = hf, where h is Planck's constant. Photons exhibit both wave-like and particle-like properties, a characteristic known as wave-particle duality.