When studying the reflection properties of such a material, it's convenient to analyze the spectrum of reflection coefficients. Their dependence on the wavelength $\lambda$ is due to the fact that the relative permittivity depends on the frequency $\omega$ of the incident light according to the microstructure of the material.
Most transparent materials in the optical range have the normal dispersion: their dielectric permittivity and refractive index increase with the light frequency $\omega$. It's a common practice to approximate this dependence with empirical formulas. An example of such an empirical formula is Cauchy's transmission equation:
\[ n(\lambda) = A + \frac{B}{\lambda^2},\]where $A$ and $B$ are some constants.
If we measure the ratio $R_p/R_s$ at the only one angle of incidence, it's impossible to obtain the refractive index $n$ unambiguously, because there are two different $n$ corresponding to the value of $R_p/R_s$. The graph below shows, how the value of $R_p/R_s$ depends on $n$ (measurements are made in air) with the angle of incidence $\Phi=60^\circ$.
In reflectometry.py on line 10-th you can choose the value of the angle of incidence $\Phi$ and plot a graph similar to the one above. You can use the cursor (values are displayed on the bottom left) to get data from it with the cursor.
There are two series of measurements $R_p/R_s$ versus $\lambda$ for fused glass $\rm SiO_2$ in air.
$\Phi=60^\circ$ $\Phi=50^\circ$ $\lambda,~\text{nm}$ $R_p/R_s$ $\lambda,~\text{nm}$ $R_p/R_s$ 378 0.01311 378 0.02522 406 0.01352 406 0.02472 434 0.01384 434 0.02437 466 0.01415 466 0.02406 499 0.01442 499 0.02376 535 0.01464 535 0.02350 573 0.01485 573 0.02327 614 0.01500 614 0.02310 658 0.01515 658 0.02288 706 0.01537 706 0.02273 756 0.01553 756 0.02260 810 0.01568 810 0.02247