WO · Waves and optics

Two individually bright waves can cancel at a point if their phases oppose.

Prerequisites: 16, 17, 18, 31, 32, 33.

• Analyse wave superposition and interference
• Use diffraction 衍射, polarisation and optical imaging
• Relate phase, frequency and dispersion

Add wave amplitudes, then calculate intensity. Intensity is proportional to squared amplitude for the same medium.

A path difference gives phase difference 相位差 2πΔ/λ. Coherent sources need stable relative phase.

For a single slit, minima satisfy a sin θ=mλ for nonzero integers m. A double slit has a different interference condition.

Geometric optics needs a stated sign convention. Wave effects matter when apertures are comparable to wavelength.

Coherent waves have a stable phase difference. Add amplitudes before squaring.

For equal individual intensities I0 and a path difference of one quarter wavelength, This is neither a maximum nor a minimum. Random relative phase removes the cross term in a long time average; stable coherence 相干性 is a physical condition, not a drawing convention.

The central single-slit maximum is not obtained by setting m=0 in the minima condition.

coherence: A stable phase relationship between waves.

diffraction: Wave spreading and interference caused by an aperture or obstacle.