Final answer:
We model light as a wave or particle based on the situation due to its wave-particle duality, with wave behavior evident in large-scale interactions and particle nature revealed at the quantum level.
Step-by-step explanation:
We must model light as a wave or a particle depending on the situation because light exhibits properties of both wave and particle models, and different experiments reveal different aspects of light's nature. The phenomenon of light behaving both as a wave and a particle is referred to as wave-particle duality. When light interacts with larger objects, it can be effectively described as a ray, which aligns with our everyday experiences with optics such as lenses and mirrors. However, when light interacts with smaller structures, especially at the atomic or quantum level, its particle nature becomes more evident, as seen in phenomena like the photoelectric effect, where it behaves as individual packets of energy called photons.
In summary, sophisticated experiments from the 20th century onwards have shown that light behaves as a wave in some instances, like diffraction and interference, while in other situations, such as energy transfer at the quantum level, it behaves like particles. This concept of wave-particle duality is a central tenet of quantum physics and is critical for understanding and utilizing the properties of light in various applications.