Huygens Principle 🤓

1️⃣ Wavefronts

A wavefront is the set of points that are all in the same phase of vibration at a given instant—imagine the bright rings that appear when a stone splashes into calm water. Energy moves perpendicularly to the wavefront, and the wavefront itself races outward with the wave’s speed 🌊.:contentReference[oaicite:0]{index=0}

• Spherical wave: A point source sends out concentric spheres, each sphere marking equal phase.:contentReference[oaicite:1]{index=1}
• Plane wave: Far from the source, a tiny patch of a sphere looks flat—so the wavefront is treated as a plane.:contentReference[oaicite:2]{index=2}

2️⃣ Huygens Principle in Plain Words

Every spot on a wavefront behaves like a fresh little source that emits its own tiny “secondary wavelet.” After a short time t, each wavelet has radius \(v\,t\). The envelope (outer tangent) of all these wavelets is the brand-new wavefront—cool, right? ✨:contentReference[oaicite:3]{index=3}

3️⃣ Forward Wave vs Backwave 🚀↩️

The same construction predicts a backward-moving envelope (backwave). Huygens boldly set the backward amplitude to zero and the forward one to a maximum—so only the forward wave survives. Later, full wave theory confirmed why the backwave really disappears, but the “forward-only” tweak does the job for now!:contentReference[oaicite:5]{index=5}

4️⃣ Plane Waves Marching Ahead ➡️

For a plane wave, draw parallels to the plane front at t = 0. After time t, the secondary wavelets overlap to give a new, shifted plane. Lines drawn perpendicular to both old and new fronts act as light rays.:contentReference[oaicite:6]{index=6}

5️⃣ Refraction with Huygens 🔍

Picture a plane wavefront AB hitting the interface PP′ between two media. While point A touches the surface, point B is still in medium 1. In the time t that B needs to reach the surface, it travels \(BC = v_{1}\,t\), where \(v_{1}\) is the speed in medium 1. The part already inside medium 2 sprouts a secondary wavelet of radius \(v_{2}\,t\). The common tangent CE is the refracted wavefront. If \(v_{2} < v_{1}\) the wavefront—and thus the ray—bends toward the normal.:contentReference[oaicite:7]{index=7}

🔑 High-Yield NEET Nuggets

1. Definition drill: Know what a wavefront is and how energy always shoots out at right angles to it.:contentReference[oaicite:8]{index=8}
2. Huygens mantra: “Every point is a secondary source” is the key to building new wavefronts quickly in ray-optics questions.:contentReference[oaicite:9]{index=9}
3. Backwave dodge: Remember the forward-only assumption—often asked as a conceptual twist.:contentReference[oaicite:10]{index=10}
4. Spherical → Plane trick: Far-field spherical waves can be treated as plane waves, simplifying calculations.:contentReference[oaicite:11]{index=11}
5. Speed matters: Refraction direction hinges on the comparison of \(v_{1}\) and \(v_{2}\); “slower medium = bend toward normal.”:contentReference[oaicite:12]{index=12}

Keep practicing, and let Huygens guide every wavefront you meet! 🚀