Before the more modern ideas on light, there was some early ides about the nature of light. In early civilizations, there were some observations on the properties of light that informed more modern theories. In particular, Euclid observed and showed that light travels in straight lines. He published this, and other observations in "Optica".
Any theory of light had two basic properties of light that been observed at the time. The first property the ability of light to be reflected off of a mirror, and the second was the property of diffraction. The theories developed by natural philosophers and physicists had to explain both of these properties, and created two major groups of theories to deal with these two properties. Unfortunately, there are more properties of light that could not be observed with the technology of the time. Despite this, the two major branches of theories come fairly close to what we understand about light today.
The two classes of theories are Particle theories, and Wave theories.
Particle Theory, in a nutshell, is the idea that light is made up of particles, much like matter is. In this sense it similar to matter, and actually comes out of early atomist thought. According to this theory, light is made up of particles that traveled in straight lines, and were emitted in all directions from a light source. The big support came from the fact that light does travel in straight lines, where waves are known to bend around objects.
Issac Newton was an early theorist of the particle nature of light, and did much a lot of work trying to prove the particle nature of light. He published his findings in book called Opticks[1] (Not to be confused with the book above, by Euclid). In this book, Newton wrote about many experiments that he performed and observations that he made about light, in particular the diffraction of light. Furthermore, he goes on to conclude that light must be a particle.
Early wave theory comes from the works of Hooke, Huygens, and Young. These thinkers were convince that light was a wave, and did some amazing work to attempt to prove their views. Hooke developed a "pulse theory" of light that suggested that the behavior of light and water was similar[2]. After Hook, Huygens published a mathematical model of the wave theory of light in the book Treaties on light[3]. Young, who had done work on sound waves earlier, also thought that light behaved like a wave, and designed an experiment to test this hypothesis. Today, this experiment is known as the dual-slit experiment, or Young's interference experiment.