Simplify the following text for a Grade 12 native English-speaking person.
Original: Newton built the first practical reflecting telescope and developed a theory of color based on the observation that a prism decomposes white light into the many colors that form the visible spectrum .
Simplified: Newton built the first practical reflecting telescope in 1668 ; he also developed a theory of light based on the observation that a prism decomposes white light into the colors of the rainbow .