Einstein's belief that light consists of particles, known as photons, instead of waves only, stemmed from his investigations into the phenomenon of the photoelectric effect and his development of the theory of light quanta or the particle-like nature of light.
The photoelectric effect refers to the emission of electrons from a material when it is exposed to light. In the late 19th and early 20th centuries, scientists struggled to explain this effect using classical wave theory. According to classical wave theory, increasing the intensity of light should increase the energy of the emitted electrons. However, experimental observations indicated that increasing the intensity of light only affected the number of emitted electrons, while the energy of each emitted electron depended on the frequency of the light.
Einstein proposed a revolutionary explanation in 1905, for which he received the Nobel Prize in Physics in 1921. He suggested that light is quantized, meaning it is composed of discrete packets of energy called photons. Each photon carries a specific amount of energy proportional to its frequency, as described by Planck's constant. The energy of an individual photon is given by the equation E = hf, where E is the energy, h is Planck's constant, and f is the frequency of the light.
Einstein's explanation successfully accounted for the observed behavior of the photoelectric effect. He proposed that when light interacts with matter, it does so through discrete interactions between photons and electrons. The energy carried by each photon determines the energy transferred to an electron, and if this energy exceeds a certain threshold, an electron can be emitted.
Einstein's ideas were supported by subsequent experiments and further developed by the field of quantum mechanics. They challenged the classical wave theory of light and contributed to the foundation of quantum physics, which revolutionized our understanding of the microscopic world. However, it's important to note that while light exhibits both wave-like and particle-like properties, the full understanding of light as both a wave and a particle required the development of quantum theory beyond Einstein's initial proposals.