The first person to use a glass prism to obtain a spectrum of sunlight was Isaac Newton. Isaac Newton conducted a series of experiments in the late 17th century, which demonstrated that white light is composed of all the colors of the visible spectrum. He achieved this by passing sunlight through a glass prism, which refracted the light into its component colors, creating a spectrum. This groundbreaking discovery laid the foundation for the study of optics and our understanding of light and color.