Huygens’ Principle helps us understand how light behaves when it moves from one medium to another—like from air to water or glass—by describing how wavefronts change direction and speed due to the properties of the new medium.
According to the principle, every point on a wavefront acts as a source of secondary wavelets that travel at the speed of light in that medium. When light enters a different medium:
- The speed of the secondary wavelets changes depending on the optical density (refractive index) of the medium.
- In a denser medium (like water or glass), light travels more slowly, so the secondary wavelets spread out more slowly.
- This difference in speed across the wavefront causes the wavefront to bend, explaining refraction.
For example, if a wavefront hits the boundary at an angle, the part that enters the denser medium first slows down, while the other part continues at its original speed until it also enters. This causes the whole wavefront to bend at the boundary.
Thus, Huygens’ Principle shows how the direction and shape of light waves change as they pass through different materials, offering a clear wave-based understanding of light propagation, refraction, and wavefront transformation across media.