Introduction:
The nature of light has long been a topic of fascination and debate among scientists. For centuries, the question of whether light behaves as particles or waves has puzzled many. It was not until the early 20th century that scientists began to unravel the mysteries of light and discovered that it exhibits characteristics of both particles and waves. This groundbreaking realization revolutionized the field of physics and laid the foundation for the development of quantum mechanics. In this article, we will delve into the main reasons that led scientists to conclude that light behaves as both particles and waves.

Historical Background:
The debate over the nature of light dates back to the 17th century when Sir Isaac Newton proposed that light consists of particles, which he called "corpuscles." Newton's particle theory of light gained widespread acceptance and dominated scientific thought for more than a century. However, in the early 19th century, Thomas Young's double-slit experiment provided compelling evidence that light exhibits wave-like behavior. Young's experiment demonstrated interference patterns, which could only be explained by the wave nature of light. This experiment marked a significant turning point in the understanding of light and sparked further investigations into its dual nature.

Wave-Particle Duality:
The concept of wave-particle duality, which states that particles like electrons and photons can exhibit both particle and wave-like properties, was formally introduced in the early 20th century. One of the key experiments that solidified this concept was the photoelectric effect observed by Albert Einstein in 1905. The photoelectric effect demonstrated that light can transfer energy in discrete packets, or quanta, known as photons. This discovery challenged the traditional wave theory of light and provided strong evidence for the particle nature of light.

Quantum Mechanics:
The development of quantum mechanics in the 1920s further confirmed the dual nature of light. Quantum mechanics, with its probabilistic interpretation of physical phenomena, reconciled the seemingly contradictory behavior of particles and waves. The famous double-slit experiment, conducted with electrons and later with individual photons, provided conclusive evidence that particles exhibit wave-like interference patterns. This experiment, along with other quantum phenomena such as the uncertainty principle, established that light behaves as both particles and waves simultaneously.

Conclusion:
In conclusion, the main reason scientists decided that light behaves as both particles and waves can be attributed to a culmination of experimental evidence and theoretical advancements. The historical experiments of Newton, Young, and Einstein, coupled with the development of quantum mechanics, reshaped our understanding of light and paved the way for modern physics. The wave-particle duality of light continues to intrigue scientists and inspire further research in the field of quantum optics. By embracing the dual nature of light, scientists have unlocked new possibilities for technological innovations and expanded our comprehension of the fundamental nature of the universe.