Voyager 1 and 2 detected a 20-fold increase in plasma density, significantly different refractive index?
The index of refraction of a plasma is the square root of the permittivity:
$$n=\sqrt{\epsilon}=\sqrt{1-\frac{\omega_p^2}{\omega^2}},\quad \omega_p\equiv\left(\frac{n_ee^2}{\epsilon_0 m_e}\right)^{\frac{1}{2}}$$
where $n_e$ is the electron number density and $e$ is the elementary charge. Note that $n$ is frequency-dependent, and so the difference in refractive index is wavelength-dependent. A plot at small $\omega$ indicates that at $\omega\gtrsim 10^5\text{ rad s}^{-1}$, this difference is fairly small, and presumably negligible for optical light, where we have $\omega\sim10^{16}\text{ rad s}^{-1}$:



The area of the spectrum where there is a significant change appears to be beyond even the interest of most radio observations.