A photon is a fundamental particle that serves as a quantum of the electromagnetic field, including visible light and radio waves. It acts as the force carrier for the electromagnetic force, existing as a discrete bundle of electromagnetic energy. Photons have no electric charge and are considered to have zero rest mass, always moving at the speed of light in a vacuum. This energy packet plays a crucial role in how light interacts with matter.
Photons from Electron Jumps
Photons are created when electrons within atoms or molecules transition between different energy levels. Electrons occupy specific energy shells or orbitals around an atom’s nucleus. When an electron absorbs energy, it moves from a lower energy state to a higher, excited state. This excited state is temporary, and the electron returns to a more stable, lower energy level.
As the electron falls from a higher energy level to a lower one, it releases excess energy as a photon. The specific energy, and thus the color or type, of the emitted photon corresponds to the energy difference between the two electron energy levels involved. This principle is fundamental to light sources like Light Emitting Diodes (LEDs), where electrons and “holes” recombine in a semiconductor, releasing photons. Fluorescent lights and neon signs also utilize electron jumps within gases to produce light. Lasers employ “stimulated emission” where an incoming photon prompts an excited electron to emit another identical photon.
Photons from Heat
Any object with a temperature above absolute zero emits thermal radiation, consisting of photons. This occurs because particles (atoms and molecules) within a heated object are in constant, agitated motion. As an object’s temperature increases, its constituent particles vibrate and move more energetically. This agitation causes electrons within the atoms to undergo acceleration and deceleration.
The acceleration and deceleration of these charged particles lead to photon emission across a continuous spectrum of wavelengths, known as blackbody radiation. Hotter objects emit more photons, which tend to have higher average energies and shorter wavelengths. For instance, a piece of metal heated to a high temperature first glows dull red, then bright orange, and eventually white or even bluish-white. Examples include the glowing filament of an incandescent light bulb, radiant heat from a hot stovetop burner, and the sun’s immense energy output.
Photons from Accelerating Charges
Charged particles, such as electrons, emit photons when they undergo acceleration, deceleration, or are deflected. This emission is a direct consequence of changing electric and magnetic fields generated by the moving charge. The acceleration influences the type and energy of the emitted photons.
One mechanism is Bremsstrahlung, or “braking radiation,” which occurs when a fast-moving electron is suddenly slowed or deflected. This often happens when an electron passes near an atomic nucleus’s strong electric field. The electron loses kinetic energy, converted into photons, commonly in the X-ray range, a process used in medical imaging. Synchrotron radiation is produced when charged particles, particularly electrons, travel at high speeds in a curved path, such as within particle accelerators or strong magnetic fields. This continuous deflection forces particles to emit a broad spectrum of photons.
Photons from Nuclear and Particle Reactions
Photons with extremely high energies originate from processes involving atomic nuclei or subatomic particle interactions. One mechanism is nuclear decay, where unstable atomic nuclei release excess energy by emitting gamma rays. Gamma rays are the highest-energy photons, resulting from transitions within the nucleus, similar to how electron jumps produce visible light. This process often occurs after other types of radioactive decay, as the nucleus settles into a more stable energy state.
Nuclear fusion and fission, processes that drive stars and nuclear reactors, also release immense energy, frequently as high-energy gamma rays. Particle annihilation is another source of high-energy photons. This occurs when a particle, such as an electron, encounters its antiparticle, like a positron. Their masses convert entirely into energy, resulting in two high-energy gamma ray photons moving in opposite directions. These reactions generate some of the most energetic photons in the universe.