James Clerk Maxwell concluded that light is an electromagnetic wave based on his theoretical work on electromagnetism and the experiments of Michael Faraday. Maxwell's equations, which he developed in the 1860s, mathematically described the behavior of electric and magnetic fields and their interplay.
Maxwell's equations showed that electric and magnetic fields can generate each other and propagate through space in the form of waves. Through mathematical analysis, Maxwell found that these waves travel at a constant speed, which he derived to be the speed of light. This led him to propose that light itself is an electromagnetic wave.
To support his theory, Maxwell also noted the similarities between the properties of light and those of electromagnetic waves. For example, both light and electromagnetic waves could be reflected, refracted, and diffracted, and they both exhibited interference patterns. These observations suggested a common underlying nature.
Maxwell's electromagnetic wave theory was further supported by experimental evidence, particularly by the work of Heinrich Hertz in the late 19th century. Hertz successfully generated and detected electromagnetic waves in the laboratory, confirming their existence and properties predicted by Maxwell's equations.
In summary, Maxwell's conclusion that light is an electromagnetic wave was based on his theoretical framework of electromagnetism and the experimental confirmation of electromagnetic wave phenomena. This discovery revolutionized our understanding of light and laid the foundation for the development of modern physics.