Light behaves like a wave when phase matters.

In the wave description, light is a coupled electric and magnetic field disturbance that propagates through space. The wave model explains interference and diffraction.

• What “electromagnetic wave” means (conceptual)
• Field oscillations and propagation direction
• Wavelength and frequency in optics
• When the wave picture becomes necessary

Interference patterns require a stable phase relationship. Coherence describes how well-defined the phase is over time and across space.

• Temporal coherence (phase stability in time)
• Spatial coherence (phase across a wavefront)
• Monochromatic vs broad-spectrum light
• Why lasers behave differently than lamps

Rays are a useful approximation when structures are much larger than the wavelength and phase effects average out. Wave optics is required when wavelength-scale features matter.

• Scale comparison: aperture size vs wavelength
• Why diffraction is a “wave signature”
• When ray diagrams still help
• Interference as phase-sensitive addition

Practice identifying when wave effects matter, using coherence ideas to predict interference, and explaining why ray optics can fail.

• Ray vs wave decision questions
• Coherence concept checks
• Wavelength-scale reasoning drills
• Short explanations: “why no fringes?”
• Exam-style wave-nature prompts