Diffraction

📋 Teacher notes

• •Diffraction is the spreading of a wave as it passes through a gap or around an edge — it happens to ALL waves (sound, water, light), not just light.
• •The amount of spreading depends on the ratio λ/a (wavelength ÷ gap width). When the gap is comparable to or smaller than the wavelength, spreading is dramatic; when the gap is much bigger than the wavelength, the wave carries on mostly straight through.
• •This is why you can hear someone through an open doorway even when you can’t see them — sound wavelengths (metres) are comparable to doorway widths, so sound diffracts strongly, while light wavelengths (hundreds of nanometres) are far too small to diffract noticeably at that gap.
• •The first minimum either side of the central bright band occurs at sinθ = λ/a — this is the standard single-slit diffraction formula at this level.
• •Diffraction is direct evidence that light behaves as a WAVE — a stream of simple particles travelling in straight lines would never spread out behind a gap.