Phase Difference in Light Waves · CAP 2002 #25
Physics · optics
Problem
Which one of the following statements is not true? The phase difference between two light waves can change (a) by reflection. (b) by the waves travelling along paths of different lengths. (c) by the waves travelling through media that have a different refractive index. (d) by being transmitted through a boundary between two media with different refractive indices.Intuitive explanation
Phase difference in optics depends on the optical path length—how far light effectively travels when accounting for the medium it's in. Reflection, different geometric paths, and different refractive indices all change optical path length and thus phase. The key insight is recognizing which process doesn't alter the relative phase: transmission through a boundary changes the wave's speed and direction, but doesn't introduce an additional phase shift the way reflection does—it's just a continuation with a new speed.
Intuitive explanation
Phase difference in optics depends on the optical path length—how far light effectively travels when accounting for the medium it's in. Reflection, different geometric paths, and different refractive indices all change optical path length and thus phase. The key insight is recognizing which process doesn't alter the relative phase: transmission through a boundary changes the wave's speed and direction, but doesn't introduce an additional phase shift the way reflection does—it's just a continuation with a new speed.