What Is Light?

Light is part of the electromagnetic spectrum, the spectrum is the collection of all waves, which include  visible light, Microwaves, radio waves (AM, FM, SW), X-Rays, and Gamma Rays.

In the late 1600s, important questions were raised, asking if light is made up of particles, or is it waves?

Sir Isaac Newton, held the theory that light was made up of tiny particles. Around the same period, Christiaan Huygens, believed that light was made up of waves vibrating up and down perpendicular to the direction of  the light travels.  Huygens' theory was the successful theory of light wave motion in three dimensions. Huygens, suggested that light wave peaks form surfaces like the layers of an onion. In a vacuum, or other uniform mediums, the light waves are spherical, and these wave surfaces advance or spread out as they travel at the speed of light. This theory explains why light shining through a pin hole or slit will spread out rather than going in a straight line.

Newton's theory came first, but the theory of Huygens, better described early experiments.

At the time, some of the experiments conducted on light theory, both the wave theory and particle theory, had some unexplained phenomenon, Newton could not explain the phenomenon of light interference, this forced Newton's particle theory in favor of the wave theory. This difficulty was due to the unexplained phenomenon of light Polarization - scientists were familiar with the fact that wave motion was parallel to the direction of wave travel, NOT perpendicular to the to the direction of wave travel, as light does.

In 1803, Thomas Young studied the interference of light waves by shining light through a screen with two slits equally separated, the light emerging from the two slits, spread out according to Huygens' principle. Eventually the two wave fronts will overlap with each other, if a screen was placed at the point of the overlapping waves, you would see the production of light and dark areas.

Later in 1815, Augustin Fresnel supported Young's experiments with mathematical calculations.
In 1900 Max Planck proposed the existence of a light quantum, a finite packet of energy which became known as the Photon.

From the early experiments and theories, both the Wave theory, and the Particle theory were correct - light is both a wave and a particle, most of the time light behaves as a wave.

Light can exhibit both a wave theory, and a particle theory at the same time.