Two Methods of Producing Coherent Sources:

1. Division of Wavefront:

• A single wavefront is split into two parts
• Example: Young's double slit, Fresnel biprism, Lloyd's mirror
• Both parts originate from the same wavefront → same phase at source

2. Division of Amplitude:

• A single beam is split by partial reflection
• Example: Newton's rings, thin film interference, Michelson interferometer, Fabry-Pérot etalon
• The transmitted and reflected beams come from the same incident beam

Coherence Types:

• Temporal coherence: Correlation over time; related to coherence length $L_c = \lambda^2/\Delta\lambda$
  • White light: $L_c \approx 1\,\mu\text{m}$ (only 1–2 fringe orders visible)
  • Sodium: $L_c \approx 30\,\text{mm}$ (~50,000 fringes)
  • Laser: $L_c$ can be meters or km (millions of fringes)
• Spatial coherence: Correlation across the wavefront width; determines fringe visibility

NEET Level: Only temporal coherence (from spectral bandwidth) is tested at NEET level.