Dual Nature of Radiation and Matter Revision Notes for NEET Physics PDF Download
🔄 DUAL NATURE OF RADIATION & MATTER · Quantum Physics
Photoelectric effect • Einstein's equation • de Broglie waves • Davisson‑Germer • NEET problems
⚡ PHOTOELECTRIC EFFECT
Emission of electrons from a metal surface when light of sufficient frequency falls on it.
Key observations:
• Instantaneous emission (no time lag)
• Existence of threshold frequency (ν₀)
• Kinetic energy ∝ frequency (not intensity)
• Photocurrent ∝ intensity (for ν > ν₀)
📐 Experimental setup
📜 EINSTEIN'S PHOTOELECTRIC EQUATION
E = hν = φ + Kmax
Work function: φ = hν₀ (minimum energy to eject electron).
Maximum KE: Kmax = hν – φ = eVs (stopping potential).
Stopping potential Vs = (h/e)ν – (φ/e).
📈 Photoelectric effect graph (Kmax vs ν)
💡 PHOTON – PARTICLE OF LIGHT
Energy: E = hν = hc/λ
Momentum: p = h/λ = E/c
Mass (relativistic) = hν/c² (zero rest mass).
🌊 de BROGLIE WAVELENGTH
Matter waves: λ = h/p = h/(mv) (for non‑relativistic).
For electron accelerated through V volts: λ = h/√(2meV) = 12.27/√V Å.
For any particle: λ = h/√(2mK) (K = kinetic energy).
🖍️ Electron wave nature – Davisson‑Germer experiment
🔬 DAVISSON‑GERMER EXPERIMENT
First experimental proof of wave nature of electrons. Diffraction pattern observed from nickel crystal.
Bragg's law: nλ = 2d sinθ (d = crystal spacing).
Confirmed de Broglie wavelength for electrons.
🎲 HEISENBERG'S UNCERTAINTY PRINCIPLE
Δx · Δp ≥ h/4π (position‑momentum). Also ΔE · Δt ≥ h/4π (energy‑time).
Simultaneous precise measurement of conjugate variables impossible.
💡 NEET TIPS & SHORTCUTS
- Threshold frequency ν₀ = φ/h; work function φ = hν₀.
- Stopping potential Vs ∝ ν, independent of intensity.
- de Broglie wavelength λ ∝ 1/√V for electrons.
- For same kinetic energy, lighter particle has longer λ.
⚠️ COMMON MISTAKES
- Confusing work function with threshold frequency.
- Using λ = h/(mv) for relativistic particles without correction.
- Assuming photocurrent depends on frequency (it depends on intensity).
- Forgetting that de Broglie waves are probability waves, not physical waves.
📌 QUICK REVISION CARD
Einstein equation: hν = φ + Kmax
Stopping potential: eVs = Kmax
Work function: φ = hν₀
Photon momentum: p = h/λ
de Broglie wavelength: λ = h/p
Electron wavelength (V volts): λ = 12.27/√V Å
Uncertainty principle: Δx·Δp ≥ h/4π