• summary_type: concept
• word_count: 140

Coherent sources maintain a constant phase difference over time — essential for sustained interference patterns. Two independent light sources are never coherent (random atomic emissions fluctuate in ~10^{-8} s). Coherent sources are created by: division of wavefront (YDSE, Fresnel biprism, Lloyd's mirror) or division of amplitude (thin films, Newton's rings). Temporal coherence relates to monochromaticity: coherence length L_c = λ^{2}/$\Delta$λ determines how many fringes are visible (N ≈ L_c/λ = λ/$\Delta$λ). Spatial coherence relates to source size: fringes disappear when source width exceeds λD_s/d. Lasers have very high temporal and spatial coherence (millions of visible fringes). For JEE, remember that interference does not violate energy conservation — energy is redistributed from minima to maxima.