Final answer:
The use of a second prism to turn a spectrum back into white light shows that white light is made up of different colors, which is a result of dispersion due to varying refractive indices for different light frequencies.
Step-by-step explanation:
When a second prism is used to recombine the separated colors of a spectrum back into white light, it demonstrates that white light is composed of colors. This occurs because a prism can separate the colors of white light due to dispersion, which means different frequencies of light have different refractive indices in a given material. Using a prism, white light is dispersed into a continuous spectrum ranging from red to violet, and when this spectrum of colors is passed through a second prism, they are refracted again in such a way that they recombine to form white light, confirming that white light is indeed made up of these constituent colors.