What condition must hold between two materials for total internal reflection to be possible at all?

1. Total internal reflection needs light travelling through a medium with a large refractive index toward a boundary with a medium of lower refractive index. 2. Beyond the critical angle for that pair, refraction cannot occur and every ray is reflected back into the first medium. 3. Going from a lower index into a higher index medium reverses the required relationship and cannot produce total internal reflection. 4. Equal indices would give no bending at all at the boundary, so there would be no critical angle to exceed. _Source: OpenStax Physics (CC BY 4.0), Ch 16 "Mirrors and Lenses", section 16.2 Refraction_