Newton’s Experimentum Crucis


The modern scientific study of color is usually attributed to Sir Issac Newton, the 17th century mathematician, physicist, astronomer and professor at Cambridge University who among other things is credited with inventing the Calculus (along with Leibniz), and developing the laws of motion and gravitation. In the realm of color, his studies date to the mid-1600’s when plague forced the closure of Cambridge University and Newton returned to his home in the English countryside. There Newton experimented with a prism he had acquired at a nearby fair to investigate issues of light and color.

At the time there were various theories about the relations between light and color. One popular theory asserted that white light was “pure” and that light became colored when it was “soiled” by interaction with objects.

To investigate this issue Newton created a laboratory in his home and set up what is referred to as his Experimentum Crucis (crucial experiment). First he darkened the room by pulling the shades. Then he poked a hole in one of the shades to allow a small beam of sunlight into the room. On a table he arranged a lens that focused the light beam on an adjacent prism. The light refracted by the prism fell on a vertical board with several holes drilled along its length.

Newton’s first finding was that the white light that fell on the prism was refracted into a rainbow of colors that could be seen on the vertical board. Given that the light was white and supposedly pure and that the glass prism was also colorless, it was difficult to see how the “soiling” theory of light and color could explain these results. On this basis Newton concluded that colors were somehow contained within the white light itself.

Next, Newton asked whether the rainbow of colors contained in white light was also contained in colored light. To address this question he placed a second prism behind the vertical board and positioned it so it received a beam of colored light that passed through one of the holes in the board. Under these conditions he found that the light exiting the second prism was the same color as the light entering. This showed that only white light had the property of containing light of different colors.

Finally, Newton conducted a third experiment where he took the beams of colored light passing through the board and focused them onto a single spot. Miraculously, the spot appeared white, the same color as the original sunbeam. This showed that the light was fundamentally unchanged by the refraction process, another refutation of the “soiling” theory.