The concept of wave-particle duality can be counterintuitive, but it arises from the behavior of entities at the quantum scale, where classical notions may not directly apply. Waves and particles are distinct concepts in classical physics, but in the realm of quantum mechanics, they are interconnected and describe different aspects of the behavior of quantum entities like light.
In classical physics, waves are indeed described as streams of particles. For example, water waves are composed of individual water molecules interacting with each other, creating a collective wave motion. This particle-like nature of waves in classical physics is often referred to as "wave packets" or "wave trains."
However, in the context of quantum mechanics, the relationship between waves and particles is different. Quantum particles, such as photons, electrons, or atoms, can exhibit wave-like behavior and particle-like behavior, depending on how they are observed or measured.
When we refer to light as a wave, we are focusing on its wave-like properties, such as interference, diffraction, and the ability to exhibit continuous and smooth behaviors. These wave-like properties can be described mathematically using wave functions, which are solutions to the wave equations that govern the behavior of quantum systems.
On the other hand, when we refer to light as a particle, we are emphasizing its particle-like properties, such as localized energy packets called photons. Photons can be absorbed or emitted as discrete quanta of energy, as observed in phenomena like the photoelectric effect or the emission of light from excited atoms.
The term "wave-particle duality" is used to capture the observation that entities at the quantum scale, such as light or electrons, can exhibit behaviors that are both wave-like and particle-like. It is a way to describe the complexity of quantum phenomena that cannot be entirely explained by classical notions of waves or particles alone.
In summary, while waves in classical physics are indeed streams of particles, the wave-particle duality in quantum mechanics refers to the coexistence of wave-like and particle-like properties in quantum entities. The terminology of calling light both a wave and a particle is an attempt to capture this dual nature and the need for both wave and particle descriptions to fully understand and describe the behavior of quantum systems.