A Level Physics Practice Exam

What condition must be true about the refractive indices for total internal reflection to occur?

1. The refractive index must be equal on both sides

2. The refractive index of the current medium must be less

3. The refractive index of the current medium must be greater

4. The refractive indices must be the same

The correct answer is: The refractive index of the current medium must be greater

For total internal reflection to occur, it is essential that the refractive index of the current medium is greater than that of the medium into which the light is trying to pass. This condition is critical because total internal reflection happens at the boundary between two media when the angle of incidence exceeds the critical angle, which is determined by the refractive indices of the two media. When light travels from a medium with a higher refractive index to one with a lower refractive index, instead of refracting into the second medium, it reflects completely back into the first medium. This phenomenon is widely observed in optical devices like fiber optics, where light signals are kept within the medium to minimize loss. Other options outline scenarios that do not support total internal reflection; for example, having equal refractive indices or the current medium being less dense would not facilitate this optical behavior, as light would either pass into the second medium or fail to meet the necessary conditions for reflection.