The sun has been the main source of light for millions of years, light travel from the sun to the earth through space till it reaches earth. The distance that light has to cover is measured by the light year which equals 9.4605284*1015 meters. In the year 1838 Friedrich Bessel (1784-1846) a German astronomer and mathematician was able to determine the light year as a measurement for the distance between the earth and the sun and distance between the planets.
Optical spectrum is the light that we can see by naked eye, the light frequency which we can see is between 400 to 790 terahertz. The light that we can see is classified to seven parts that are visual when the light is passed through a prism or through a rainbow. The seven colors of light were first mentioned in the Samba Purana (an old text) in the Ancient India that might go back to 3500 years ago. In the 13th century, Roger Bacon (1214-1294) an English philosopher noticed the optical spectrum in a glass of water.
In the 17th century, Isaac Newton (1642-1727) the famous astronomer, mathematician, physicist and philosopher explained the visible spectrum (optical spectrum) and discovered that the prism can show the seven colors of the light. Newton named the seven colors, red, orange, yellow, green, blue, indigo and violet.
Johann Wolfgang von Goethe (1749-1832) the German physicist observed that the Newton theory is only true in the narrow beams, but in the wide beams the the light will not split to seven colors, but only to reddish yellow on one edge and blue in the other edge and white in between.
In the 19th century, William Herschel (1738-1822) a German-British astronomer was able to discover the Infrared which have a frequency from 1 to 400 terahertz, that is why it is unseen by the naked eye.
In the 19th century, Johann Wilhelm Ritter (1776-1810) a German chemist and physicist was able to discover the Ultraviolet which have a frequency higher than the white light that we can see by our eyes that is why it is cannot be seen.
In the year 1802, Thomas Young (1773-1829) the English scientist, proved that the light is consisted of waves, he produced the measure of each color wavelength.