Physicist's explanation of the white area behind the prism: White light is shone through a prism on a screen S. Each vertical row of letters on the screen marks a monochromatic spectrum produced by a single ray of white light incident on the prism from the left. This is the theoretical spectrum that physics talks about, when using the language of ray optics. It is a simplified ray optical translation from the language of different wavelengths of electromagnetic radiation. The uppermost ray of white light is resolved into colors behind the prism, red on top (R), then yellow under it (Y), then green (G), then blue (B) and finally at bottom the color violet (V). The next ray of white light, just under the first one, is refracted a bit lower forming similar sequence of colors a bit lower than the first one, and so on. Colors from different rays of white light will therefore overlap each other on the screen. A horizontal row, where all the letters (colors) are present, produces white light again. The visual color at every point in the spectrum on the screen is produced from various monochromatic lights originating from different rays of white light on the left.

Thus goes the reasoning. It is problematic because the theoretical spectrum is nowhere to be seen. It is unobservable. It is understandable only by the theoretical reasoning with wavelengths of electromagnetic radiation. This way of explaining the appearance of white in the middle of the spectrum is done by virtue of the theory itself. And here exactly lies the problematic. In the physical explanation of the visual spectrum one ends up denying the reality of the visual spectrum by taking the same observational phenomenon (the visual spectrum) as a theoretical starting point.

Newton had maintained that his experiments show by observation, ”as a fact” that white light consists of colored lights, which are separated by the prism in refraction. His argumentation for the theory is based on observation. If observation shows something else, then the theory may not be validated by these experiments. Newton is well aware of this and that is why white light after the refraction is problematic and he does not mention it. When it is mentioned and explained by his followers, it is done so by virtue of the theory, not by observation of facts. One cannot use the theory of ”white light being composed of colored lights” to explain the white area in spectra, because precisely these experiments were supposed to prove the theory valid in the first place, and this by observation.