Is it sort of how oil makes paper transparent? Impregnates air gaps to suppress index mismatch scattering? I'm just offering this casually, someone please correct me if I'm off.
If you scroll down a lot, the post goes into details.
I have to admit, it's a bit hard to follow the exact details as a non-physicist on a lazy sunday morning... but you change the refractive index by uhh... something related to the polarization and Maxwell's equations
It's a kind of impedance matching. The (almost) same happens with sound in our ears (and the 3 tiny bones). Making the audio machinery "transparent to sound" (by impedance matching).