NCEA Level 3 Waves Practice Test

The critical angle is defined as the angle of incidence at which light, when passing from a more optically dense medium to a less optically dense medium, is refracted at an angle of 90 degrees relative to the normal. This scenario results in total internal reflection occurring for any incidence angle greater than the critical angle.

When light hits the boundary at an angle greater than this critical angle, it will reflect entirely back into the denser medium rather than refracting into the less dense medium. This phenomenon is fundamental in optics, particularly in fiber optics and other applications where controlling light behavior is essential. Thus, the critical angle specifically marks the threshold beyond which light is completely reflected, making Option B the correct choice.

The other options do not accurately capture the essence of the critical angle concept. For instance, while it can relate to angles of incidence and reflection, only the definition focused on the transition to total internal reflection aligns with how the critical angle is conventionally defined in optics.