Wave Intensity
Overview
Intensity I = power / area = P/A (unit: W/m²). For a point source radiating equally in all directions: I = P/(4πr²) — intensity follows an inverse-square law (I ∝ 1/r²). Intensity ∝ amplitude²: I ∝ A². Doubling amplitude quadruples intensity. Energy transmitted per second by a wave per unit area. Exam tip: when comparing intensities at different distances or amplitudes, use ratios: I₂/I₁ = A₂²/A₁² = r₁²/r₂².
Intensity — power per unit area
Intensity I = P/A, where P is power (W) and A is the area over which it spreads (m²). Unit: W/m². For a point source emitting in all directions, the power spreads over a sphere: I = P/(4πr²). This gives the inverse square law: I ∝ 1/r² — doubling the distance reduces intensity by a factor of 4. Also: I ∝ A² (intensity proportional to amplitude squared).
- ⚠Using I ∝ 1/r instead of I ∝ 1/r² (it's inverse square, not inverse)
- ⚠Confusing intensity (power per area) with amplitude — they are related by I ∝ A²