What are coherent sources? Why are they needed for sustained interference? Write the conditions for sustained interference of light.

How to lay out the answer. Follow the frame, then write it in your own words.

1 · Define coherent sources. Two sources of the same frequency with a constant phase difference.
2 · Explain why they are needed. Only then is φ fixed at every point, so the intensity 4I₀ cos²(φ/2) stays fixed: steady fringes.
3 · Contrast with incoherent sources. φ changes randomly, cos²(φ/2) averages to ½, and I = 2I₀ everywhere: no fringes.
4 · List the conditions.
- Sources from one parent source (coherent)
- Monochromatic light of one frequency
- Nearly equal amplitudes
- Narrow slits close together
5 · Real life. Two car headlights never form fringes on the road.
6 · Close in one line. So, sustained interference needs coherent sources.

Scoring tip: Write I = 4I₀ cos²(φ/2) for coherent and I = 2I₀ for incoherent; the two formulas earn the marks.