Newton's Blue Light Experiment

by
Philip Bell
KIE Developer

(Picture linked from Newton Institute)

Newton (1642-1727) was a scientist and mathematician who is perhaps most famous for his ideas about how objects move. But he also spent quite a bit of time thinking about light.

Newton knew that Aristotle and Kepler thought that light and color were not directly related. They thought color came from white light being changed by an object's color. But, he also knew that Grimaldi had suggested that light and color were connected somehow. As he was investigating these two alternatives, Newton performed many light experiments, including the following which turned out to be crucial in his explanation of light.

Newton's Blue Light Experiment

(Picture linked from The Exploratorium)

To find out, Newton took this one step further by selecting out only blue-colored light coming from one prism and passing it through a second one. He thought this would help decide how prisms produced the different colors. I wanted to try Newton's experiment myself. The picture below shows how I set it up. Light from the first prism is coming from the left and hitting the two sheets of paper.

The blue portion goes through the slit and goes through the second prism, creating a blue band on the far sheet of paper. Here's a better picture of what it looked like in the darkened room:

Newton's Explanation For This

Newton noticed that when he passed only the blue portion of the light into the second prism, only blue light came out. According to Aristotle and Kepler, the blue light should have been changed more by passing it through another prism -- but it wasn't! The light entering the second prism looked the same as the light coming out. Newton explained this by thinking of the blue light as a particular kind of light. Looking back to the first prism which was producing a rainbow of colors using only white sunlight, Newton proposed the idea that white light is actually a mixture of different colors of light.