1.1 Light in the Ancient and Modern Era
In the beginning, God said, "Let there be light," and there was light. God saw that the light was good. However, ever since the sixth day of the heavens and the earth, the human understanding of the nature of light has been keenly contested. The very earliest ideas are lost to history, no doubt influenced by natural phenomena such as sunlight, starlight, lightning, and fire. The first recorded thoughts on light were offered by the ancient Greeks. In the sixth century BCE, Pythagoras (570-495 BCE) reasoned that sight required visual rays to leave our eyes and shine upon an object. Expanding on this notion, the philosopher Empedocles (490-430 BCE) believed the eye to be composed of the four basic elements (water, earth, air, and fire), with the fire of the eye facilitating vision. An outcome of this notion is the ability to see in the dark; to counter this difficulty, Plato (428-348 BCE) supposed that the fire of the eye mixes with daylight to enable a link between man and the external world. Around 300 BCE, the mathematician Euclid (325-265 BCE) provided a geometric description of visual rays and proclaimed that "light travels in a straight line." Much later, in 499 CE, the Indian astronomer Aryabhata (476-550) recognized that objects are seen by reflected light. In his virtually unknown work, he wrote that celestial objects do not shine through their own intrinsic effulgence but through the light from the Sun that they reflect. The Arab mathematician and physicist Ibn Al-Haytham (965-1039) noted that extremely bright light injures the eye, and he concluded that "[…] light comes to the surface of the eye from the light of the visible object."
In the early modern period, the philosopher and mathematician Rene Descartes (1596-1650) postulated the idea of luminiferous (light-bearing) aether, i.e., a medium for the propagation of light that is imperceptible by human senses. He suggested that the speed of light is infinite, traveling from one place to another in an instant; however, in 1676, this idea was dispelled by the astronomer Ole Romer (1644-1710), who estimated a finite speed of light by observing the eclipses of Jupiter's moon Io. Christiaan Huygens (1629-1695) and Robert Hooke (1635-1703) supported the idea of light as a vibratory motion of the aether, much like ripples propagating on the surface of water. However, most scientists around that time accepted the corpuscular theory of light advocated by Isaac Newton (1643-1727), primarily owing to the great reverence placed on Newton's accomplishments and the deficiencies of experimental apparatus in the 18th century. According to the theory of corpuscles, luminous objects eject tiny particles of light that are governed by Newton's laws of motion. The color was thought to vary with its size, where red is the largest particle and violet the smallest. Although Newton accepted the concept of an aether, he did not believe that it acted as a medium for propagating light.