When does total internal reflection occur?

Total internal reflection occurs when light travels from a medium with a higher refractive index (a slow medium) to a medium with a lower refractive index (a fast medium). This phenomenon can only happen if the angle of incidence is greater than the critical angle, which is a specific angle of incidence at which light is refracted at 90 degrees along the boundary. If the light hits the boundary at an angle greater than this critical angle, rather than refracting into the second medium, it reflects back entirely into the first medium. This principle is crucial in fiber optics and numerous applications where it's essential to keep light confined within a medium without losing it due to refraction.

When light enters a fast medium from a slow one, refraction occurs instead of reflection, and light penetrates the second medium. Similarly, striking a reflective surface leads to reflection based on the law of reflection and does not involve total internal reflection. Entering a vacuum represents another instance of refraction but does not pertain to the total internal reflection concept since there is only one boundary involved and no critical angle consideration.