Wave Optics Revision Notes for NEET Physics PDF Download
🌊 WAVE OPTICS · Interference, Diffraction & Polarization
Huygens principle • Young's double slit • Fringe width • Diffraction • Polarization • NEET problems
🔮 HUYGENS PRINCIPLE
Every point on a wavefront acts as a source of secondary wavelets; the new wavefront is the envelope of these wavelets.
Used to explain reflection, refraction, diffraction, and interference.
📐 Spherical wavefronts
🌈 YOUNG'S DOUBLE SLIT EXPERIMENT (YDSE)
Two coherent sources produce interference pattern of alternating bright and dark fringes.
Path difference: Δx = d sinθ ≈ d y/D
Constructive (bright): Δx = nλ → y = nλD/d
Destructive (dark): Δx = (2n–1)λ/2 → y = (2n–1)λD/(2d)
Fringe width (β): β = λD/d
📈 Interference pattern & intensity distribution
📏 DIFFRACTION (Single Slit)
Bending of light around obstacles. For single slit of width a:
Minima (dark): a sinθ = nλ, n = ±1, ±2,…
Central maxima width: 2λD/a
Intensity distribution: I = I₀ (sinβ/β)², β = (πa sinθ)/λ
📊 Single slit diffraction pattern
🔆 POLARIZATION
Transverse nature of light. Unpolarized light becomes polarized after passing through a polaroid.
Malus' law: I = I₀ cos²θ (θ = angle between polarizer & analyzer).
Brewster's angle: tan ip = n₂/n₁ (reflected light is completely polarized).
At ip, reflected and refracted rays are 90° apart.
🔘 Polarization by reflection
🔬 RESOLVING POWER
Rayleigh's criterion: Two images are just resolved when central max of one coincides with first min of the other.
For telescope: R = a/(1.22λ) (a = aperture diameter).
For microscope: R = 2n sinθ / λ (n = refractive index, θ = half angle).
💡 NEET TIPS & SHORTCUTS
- YDSE fringe width β ∝ λ; if wavelength increases, fringe width increases.
- In diffraction, central maxima is twice as wide as secondary maxima.
- Polarization proves light is a transverse wave.
- For coherent sources, phase difference = (2π/λ) × path difference.
⚠️ COMMON MISTAKES
- Confusing diffraction and interference conditions.
- Using YDSE formula for diffraction minima (different n).
- Forgetting that in polarization, only transverse component is transmitted.
- Assuming interference pattern disappears if sources are not coherent.
📌 QUICK REVISION CARD
YDSE bright fringes: y = nλD/d
YDSE dark fringes: y = (2n–1)λD/(2d)
Fringe width: β = λD/d
Single slit minima: a sinθ = nλ
Malus law: I = I₀ cos²θ
Brewster angle: tan ip = n₂/n₁
Resolving power (telescope): a/(1.22λ)