Main Answer:
The photoelectric effect was used to argue that light behaves as a particle.
Step-by-step explanation:
The photoelectric effect, observed by Albert Einstein in 1905, demonstrated that light could be thought of as discrete packets of energy called "photons."
In the photoelectric effect, light shining on a material surface causes the emission of electrons. The key observation was that the energy of the emitted electrons depended on the frequency of the light, not its intensity.
Classical wave theory couldn't explain this phenomenon, but Einstein proposed that if light were treated as particles (photons), each carrying a specific amount of energy, the observations could be explained.
This experiment played a pivotal role in establishing the dual nature of light—both particle and wave—leading to the development of quantum mechanics.
Therefore, the photoelectric effect was a critical experiment used to argue that light exhibits particle-like behavior.