As man emerged from his primeval state, he became conscious of the formation of shadows, the color of the daytime sky, the spectacle of the rainbow, and the motions of the stars at night and the sun in the day. Thus, he began his awareness of light. Initially, he could seek to discern the nature of light only by noting its rectilinear motion. But this soon led to observations which tended to clarify the understanding of mechanisms responsible for these phenomena.
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