Refractive index in optics is impedance, and mismatches cause reflection at interfaces.

Evidence

Impedance matching plays a role in many domains. In the preceding example, the coins were a “medium” for the propagation of the translational energy possessed by the first coin. Light is similar: it propagates nicely through transparent media, such as air and glass, but these do not have the same impedance. The impedance (called refractive index in the case of light) has a mismatch passing from air to glass, with the result that some light will reflect at the interface, whether it is coming from air to glass or vice versa. If a coating can be found with intermediate impedance, it can break up the impedance mismatch into two smaller steps, with the result that less light will be reflected and more transmitted. This is the principle of antireflection-coated eyeglasses and camera lenses. The coating works better for some colors (wavelengths) than others; this explains the color sheen often seen on coated optics. — Eric J. Heller, Why You Hear What You Hear- An…, 74

Categorization

• DEFINE: Unifies optical refractive index with mechanical impedance, a surprisingly cross-domain unifying framework.
• XREF: Ties to antireflection coating physics; connects to broader wave phenomena across acoustics and optics.
• SEED: Could be an essay on impedance matching as a universal principle across sound, light, and energy transfer.