Observe the spectrum that results from passing white light through a prism
White light, whether it comes from the Sun or an incandescent lamp, when it changes from one propagation medium to another, undergoes refraction, that is, it undergoes a change in the propagation speed. This change of propagation medium causes white light to decompose into infinite rays of monochromatic light, known as the seven colors of the rainbow. This physical process therefore constitutes the decomposition of white light .
Although we know that white light is made up of an infinity of colors, such colors of light do not behave in the same way when they change from one propagation medium to another. The light that comes closest to normal is violet; next are the colors: indigo, blue, green, yellow, orange and red. The colors that make up white light are called the spectrum of light.