• Tags: coherence, interference, conditions
• Difficulty: Foundation

Coherent sources maintain a constant phase difference over time. This is essential for sustained interference patterns. Two independent light sources (like two bulbs) are never coherent because their emissions are random. Methods to create coherent sources include: division of wavefront (YDSE, Fresnel biprism, Lloyd's mirror) and division of amplitude (thin film interference, Newton's rings). Temporal coherence relates to monochromaticity — a perfectly monochromatic source has infinite coherence time. Spatial coherence relates to the finite size of the source — a point source has perfect spatial coherence. Laser light is highly coherent (both temporally and spatially), making it ideal for interference experiments.