Final answer:
Before 1900, scientists thought light behaved solely as a wave. Yet, experiments in the early twentieth century showed light also had particle-like traits, leading to the wave-particle duality concept.
Step-by-step explanation:
Prior to 1900, scientists indeed thought light behaved solely as a wave, displaying wave properties like diffraction and interference. This view was established by the work of Thomas Young and James Clerk Maxwell, among others. However, experiments in the early twentieth century revealed that light also exhibits particle-like characteristics, leading to the acceptance of the wave-particle duality concept, which combines the corpuscular hypothesis and the wave theory into a new framework for understanding light.