Interference and Diffraction of Waves and Light
Why two waves can add up to darkness, and why light bends around edges
Big Idea: When two waves overlap, their displacements simply add. Where a crest meets a crest the wave grows louder or brighter; where a crest meets a trough they cancel and you get silence or darkness. This overlapping is called interference. Waves also bend around obstacles and spread out after passing through narrow openings — that bending is diffraction. Together these two effects prove that light is a wave, and they explain the colours of a soap bubble, the rainbow sheen of a CD, and why you can hear a friend around the corner of a building.
By the end of this topic you will be able to:
- state the principle of superposition and apply it to two overlapping waves,
- explain what coherent sources are and why they are needed for a stable pattern,
- use the path-difference conditions for a maximum (Δd = kλ) and a minimum (Δd = (k + ½)λ),
- describe Young's double-slit experiment and find the fringe width Δx = λL/d,
- explain diffraction and the diffraction grating relation d sin θ = kλ,
- recognise everyday examples of interference and diffraction.