Huygens’ Principle explains the propagation of light waves by imagining that each point on a light wavefront acts like a tiny source of new wavelets. These secondary wavelets spread out in all directions at the same speed as the original light wave. After a short time, the envelope or outer boundary of all these wavelets forms a new wavefront. This process repeats continuously as the wavefront moves forward.
Here’s how it explains propagation:
- When light travels in a straight line through a uniform medium, the secondary wavelets from each point on the wavefront combine to form a new, forward-moving wavefront. This explains how light advances smoothly through space.
- When light encounters a new medium (like from air to water), the speed of wavelets changes due to the change in refractive index. This causes the new wavefront to bend, explaining refraction.
- When light passes through a narrow opening or around an obstacle, wavelets spread into the shadowed region, explaining diffraction.
- In reflection, the wavelets from a wavefront striking a reflective surface generate new wavelets that move away at an angle equal to the incident angle, forming the reflected wavefront.
Thus, Huygens’ Principle gives a simple yet powerful way to understand how light waves move, bend, and spread, confirming their wave-like behavior.