Newton’s Crucial Experiment Recomputed: Crown vs. Dense Flint Prisms

In 1672, Isaac Newton communicated his landmark paper to the Royal Society detailing the Experimentum Crucis. By passing a single dispersed spectral color from a first prism through an aperture into a second prism, Newton proved that color was not a quality imparted by glass, but an innate, immutable property of light itself.

Over 350 years later, we re-examine this experiment using digital spectral solvers.

Abbe Number and Chromatic Dispersion

Different glass chemistries exhibit radically different dispersion slopes. The Abbe number V_d quantifies this behavior:

V_d = \frac{n_d - 1}{n_F - n_C}

Where:

• n_d is the refractive index at the Helium d-line (587.56 nm)
• n_F is the Hydrogen F-line (486.13 nm)
• n_C is the Hydrogen C-line (656.28 nm)

| Substrate | n_d | Abbe V_d | Dispersion Character | | :--- | :--- | :--- | :--- | | N-BK7 Crown | 1.5168 | 64.17 | Low dispersion, high transmission | | SF11 Flint | 1.7845 | 25.76 | High dispersion, broad rainbow band | | Fused Silica | 1.4585 | 67.82 | Deep UV to near-IR transparency |

In the Solis interactive apparatus, users can toggle between N-BK7 and SF11 to observe the dramatic broadening of the 400nm–700nm projection angle.