Every time you open your eyes, the first thing you see is light. In fact, it's the only thing we see. Strangely, light remained largely a mystery until only a couple centuries ago with the work of pioneering scientists.
For years, scientists debated the precise nature of light. Isaac Newton proposed that light was composed of countless tiny particles, what we now call photons. Other scientists, such as Christiaan Huygens, proposed that light was in fact a wave. Due to Newton's popularity at the time, the particle model was accepted by most scientists. This all changed when Thomas Young performed the famous double slit experiment. When light passed through a two slit panel, it produced what's called an interference pattern. An interference pattern can only be produced by waves, and thus the wave model of light was accepted. Today we know that light is in fact made up of particles called photons, yet it also behaves like a wave.
Another breakthrough came in the 1800s. In the 1800s, Michael Faraday proposed the concept of the "field" or "action at a distance." Objects, he proposed, could interact with each other without touching via a field. An electric charge creates an electric field; a 3D region of influence surrounding an object. Faraday also showed that the electric force and the magnetic force were somehow related. A magnet was able to produce an electric current. Unfortunately, Faraday lacked a formal education and was not able to mathematically combine electricity and magnetism. Another scientists would be the one to figure out the relationship between these two forces.
In 1865, James Clerk Maxwell proposed the Electromagnetic Theory. Maxwell proposed that a changing electric field produces a changing magnetic field and that the interaction between these two fields propagates as a wave through space. Electromagnetic waves, he predicted, moved at a speed of around 300,000 km/s. This speed also happens to be the speed of light. This fact led to a remarkable breakthrough: light is an electromagnetic wave.