Huygens’ Principle explains refraction by showing how a wavefront changes direction when it moves from one medium to another with a different speed of light (i.e., different refractive index).
Here’s how it works:
When a wavefront of light passes from one medium (like air) into another (like water or glass), the speed of light changes. According to Huygens’ Principle, every point on the wavefront acts as a source of secondary wavelets. These wavelets spread out in all directions, but their speed depends on the medium they are in.
At the boundary between two media:
- As soon as the wavefront hits the interface, points in the slower medium generate wavelets that travel more slowly.
- The rest of the wavefront, still in the faster medium, produces wavelets that continue moving faster.
- This difference in speed causes the wavefront to tilt, forming a new wavefront in the second medium that is bent compared to the original one.
This bending of the wavefront results in the change in direction of the light ray, which we observe as refraction.
Significance:
- Huygens’ Principle gives a wave-based explanation for Snell’s Law.
- It shows that the bending of light is due to the different speeds of wavelets in different media.
- It reinforces the idea that light behaves as a wave, not just a ray.
So, refraction is the natural outcome of how secondary wavelets spread at different speeds across a boundary, changing the shape and direction of the advancing wavefront.