In the late 19th century, scientists struggled to explain how hot objects emitted light of different colors. The prevailing theory predicted a spectrum (distribution of colors) that didn’t match reality.

First proposed by Max Planck in 1900 to explain the blackbody radiation spectrum, he proposed a revolutionary idea. He suggested that light wasn’t emitted continuously, but in discrete packets of energy called quanta. This was a radical departure from the classical view of light as a wave.

A few years later, in 1905, Albert Einstein applied Planck’s concept of quanta to explain the photoelectric effect. This phenomenon observed that light hitting a metal surface could eject electrons, with the energy of the ejected electrons depending on the light’s frequency, not its intensity (as classical theory predicted).

Following these breakthroughs, physicists like Gilbert Lewis (who coined the term “photon” in 1926) and Arthur Compton (who explored photon-electron collisions) continued to refine the understanding of the photon.

The development of quantum mechanics solidified the wave-particle duality of light. Photons exhibit both wave-like and particle-like behavior depending on the experiment.

State of the Art

In contemporary physics, photons are fundamental particles that exhibit dual wave-particle behavior, serving as carriers of electromagnetic radiation and playing a central role in the interactions of light with matter.

Described by quantum mechanics, photons possess both wave-like and particle-like properties, allowing them to propagate through space as electromagnetic waves while also exhibiting discrete energy quantization characteristic of particles. The wave-particle duality of photons is exemplified by phenomena such as the double-slit experiment, where they demonstrate interference patterns characteristic of waves, yet interact with detectors as discrete entities.

Massless and electrically neutral, photons travel at the speed of light in a vacuum and carry energy proportional to their frequency, as described by Planck’s relation, E=hf. This energy-carrier property of photons underlies their role as the fundamental particles of light and electromagnetic radiation across the electromagnetic spectrum, from radio waves to gamma rays.

Photons are foundational to modern optics, quantum mechanics, and particle physics, embodying the concept of wave-particle duality elucidated by Erwin Schrödinger. Within the standard model of particle physics, photons are considered elementary particles that interact via electromagnetic forces, contributing to our understanding of fundamental interactions in nature.

While traditionally viewed as particles, they also exhibit wave-like behavior under certain conditions, underscoring the intricate nature of quantum objects and the nuances of their interactions in the fabric of reality.