The angle of incidence has a critical effect on whether total internal reflection (TIR) occurs:
1. Less than the Critical Angle:
- When the angle of incidence is less than the critical angle, light passes from the denser medium (higher refractive index) into the rarer medium (lower refractive index).
- Some of the light is refracted out, and some may be partially reflected internally, but TIR does not occur.
2. Equal to the Critical Angle:
- When the angle of incidence is equal to the critical angle, the refracted light travels along the boundary between the two media.
- This is the threshold condition for TIR to begin.
3. Greater than the Critical Angle:
- When the angle of incidence is greater than the critical angle, total internal reflection occurs.
- All the incident light is completely reflected back into the denser medium.
- No light is transmitted into the second medium.
Key Points:
- Higher angles (beyond the critical angle) ensure more effective TIR.
- The critical angle depends on the refractive indices of both media.
- TIR can only happen when light moves from higher to lower refractive index.
Thus, the angle of incidence must be carefully controlled in devices like optical fibers and prisms to ensure that TIR occurs and light remains confined.