What significant concept is associated with the work of Max Planck and Albert Einstein?

The significant concept associated with the work of Max Planck and Albert Einstein is that light behaves as both a particle and a wave. This dual nature of light is a cornerstone of quantum mechanics. Max Planck's introduction of the idea of energy quanta laid the groundwork for this understanding. He proposed that electromagnetic radiation is emitted or absorbed in discrete units called quanta, or photons, which can exhibit particle-like properties.

Albert Einstein further expanded on Planck's work through his explanation of the photoelectric effect, where he demonstrated that light can be seen as composed of particles (photons) that can knock electrons out of materials. This was pivotal in establishing the concept of wave-particle duality, which posits that light—and indeed all matter—exhibits both wave-like and particle-like characteristics.

This foundational principle radically changed the way scientists view light and matter, leading to significant advancements in quantum theory and its applications. The other choices, while important in the realm of physics, do not directly pertain to the core contributions made by Planck and Einstein regarding the nature of light.