Diffraction 🌈

Light doesn’t always travel in neat, straight lines. When it meets a tiny obstacle or a narrow opening, it bends around the edges and fans into the “shadow” region. That bending and spreading out is called diffraction — a wave property shared by sound, water and even matter waves. Diffraction is the very reason a CD flashes rainbow colours and why microscopes can’t zoom in forever — their resolution is limited by the wave nature of light. :contentReference[oaicite:0]{index=0}

1 ️⃣ Single-Slit Pattern

  • A narrow slit of width a lit with monochromatic light acts like a line of tiny secondary sources. Their waves overlap on the screen and create a broad, bright central band with fainter, alternating dark and bright fringes on both sides. 😊 :contentReference[oaicite:1]{index=1}
  • Minima (dark fringes) appear when the path difference makes the waves cancel out:
    \(a\sin\theta = n\lambda,\quad n = \pm1, \pm2, \dots\) :contentReference[oaicite:2]{index=2}
  • Secondary maxima (bright fringes) occur halfway between the minima:
    \(a\sin\theta = \bigl(n+\tfrac12\bigr)\lambda\) :contentReference[oaicite:3]{index=3}
  • The central maximum sits at \(\theta = 0\) and is twice as wide as every other bright band. Its edges lie where \(a\sin\theta = \lambda\), so its angular width is \(\Delta\theta = \dfrac{2\lambda}{a}\). A slimmer slit or a longer wavelength stretches this bright patch. :contentReference[oaicite:4]{index=4}

2 ️⃣ Why Does It Happen? 🔍

Think of the slit as being sliced into many tiny sections. Each slice sends out a wave in step with its neighbours. At any screen point, these little waves arrive with slightly different phases. Where they march in step, we see a bright fringe; where they step out of line, they wipe each other out and form a dark fringe. :contentReference[oaicite:5]{index=5}

3 ️⃣ Interference vs. Diffraction 🤔

With just a couple of sources (like two slits) we usually say “interference.” With a crowd of sources (all the points across one slit) we often say “diffraction.” But, as Richard Feynman cheekily pointed out, the two are really the same wave-addition game — only our vocabulary changes! :contentReference[oaicite:6]{index=6}

4 ️⃣ Try It Yourself! 🛠️

  1. Hold two razor blades so their edges form a narrow, straight slit (see Fig. 10.16 for the grip). :contentReference[oaicite:7]{index=7}
  2. Look at a clear-glass bulb with a straight filament through this slit. The filament plays the role of a “light line.” Bright and dark bands should pop into view on your retina. 🎉
  3. Pop on a red or blue filter. Red light (longer λ) spreads wider; blue light bunches tighter — a colourful confirmation of \(\Delta\theta = 2\lambda/a\)!
  4. Avoid direct sunlight — it’s too broad an angle and risks eye damage.

5 ️⃣ Energy Shuffle ⚡

Light energy isn’t lost; it just moves. Where waves cancel (a dark fringe), the energy turns up at neighbouring bright fringes. Total energy stays the same, cheering on the principle of conservation of energy. :contentReference[oaicite:8]{index=8}

High-Yield Ideas for NEET 📚

  • Minima condition: \(a\sin\theta = n\lambda\) (integral order).
  • Maxima condition: \(a\sin\theta = \bigl(n+\tfrac12\bigr)\lambda\) (half-integral order).
  • Central-band width: \(\Delta\theta = 2\lambda/a\) — key to questions on slit width vs. wavelength.
  • Diffraction-limited resolution: smaller λ or larger aperture sharpens images of telescopes, microscopes and even the eye. :contentReference[oaicite:9]{index=9}
  • Interference vs. diffraction language: few sources → “interference,” many sources → “diffraction.” (Feynman’s handy rule of thumb.) :contentReference[oaicite:10]{index=10}