How does quantum mechanics explain the behavior of light?

Quantum mechanics provides a comprehensive framework for understanding the behavior of light. Traditionally, light was described as a wave in the electromagnetic spectrum, characterized by its wavelength and frequency. However, certain phenomena, such as the photoelectric effect, could not be explained by the wave theory alone. This led to the development of the quantum theory of light, which introduced the concept of photons.

The Dual Nature of Light

Quantum mechanics reveals that light exhibits both wave-like and particle-like properties, a phenomenon known as wave-particle duality. When light interacts with matter, it behaves as if it is composed of discrete packets of energy called photons. Each photon carries a quantum of energy proportional to its frequency, as described by the equation \(E = hf\), where \(E\) is the energy, \(h\) is Planck's constant, and \(f\) is the frequency of the light.

Photoelectric Effect

The photoelectric effect demonstrates the particle nature of light. When light of a certain frequency shines on a metal surface, it can eject electrons from the metal. Classical wave theory could not explain why light below a certain frequency, regardless of its intensity, failed to eject electrons. Quantum mechanics resolved this by proposing that photons must have enough energy (related to their frequency) to overcome the binding energy of electrons in the metal.

Quantum Electrodynamics (QED)

Quantum Electrodynamics (QED) is the quantum field theory that describes how light and matter interact. In QED, light is represented by photons, and their interactions with charged particles, such as electrons, are governed by the exchange of these photons. This theory accurately predicts and explains a wide range of optical phenomena, including reflection, refraction, and diffraction.

Superposition and Interference

Quantum mechanics also explains the interference patterns observed in experiments like the double-slit experiment. When light passes through two slits, it creates an interference pattern on a screen, suggesting wave-like behavior. However, when the experiment is conducted with individual photons, each photon still contributes to the same interference pattern over time. This phenomenon is explained by the principle of superposition, where a photon exists in a superposition of all possible paths until it is observed.

Quantum Entanglement

Quantum mechanics introduces the concept of entanglement, where particles become interconnected such that the state of one particle instantaneously affects the state of another, no matter the distance between them. This has profound implications for the behavior of light, particularly in experiments involving entangled photons, where measurements on one photon influence the properties of its entangled partner.

Conclusion

Quantum mechanics offers a detailed and robust explanation for the behavior of light, encompassing both its wave-like and particle-like properties. By introducing concepts such as photons, superposition, and entanglement, it provides a deeper understanding of light's interaction with matter and the fundamental principles governing these interactions. This quantum framework not only resolves classical paradoxes but also underpins modern technologies and ongoing research in physics.