The refractive index difference between two media directly determines whether total internal reflection (TIR) can occur and at what angle.
- Higher to Lower Refractive Index: TIR only occurs when light travels from a medium with a higher refractive index (like glass or water) into a medium with a lower refractive index (like air). If the light moves in the opposite direction (low to high), TIR cannot happen.
- Critical Angle Dependence: The critical angle is the minimum angle of incidence at which TIR occurs. It depends on the ratio of the refractive indices of the two media. A larger difference between the indices results in a smaller critical angle. This means TIR will occur more easily (at a smaller incident angle).
- For example, light traveling from diamond (n ≈ 2.42) into air (n ≈ 1.00) has a much smaller critical angle than light going from water (n ≈ 1.33) into air.
- Efficiency of TIR: A greater refractive index contrast increases the likelihood of light staying confined within the denser medium due to more frequent TIR. This is why optical fibers are designed with a significant difference between the core and cladding refractive indices to ensure strong and continuous internal reflection.
In essence, the larger the refractive index difference (with light going from high to low), the more favorable the conditions are for total internal reflection to occur efficiently.