• Tags: Lloyd-mirror, Fresnel-biprism, interference
• Difficulty: Moderate

In Lloyd's mirror, a single slit source and its virtual image (formed by reflection from a flat mirror) act as coherent sources. Key difference from YDSE: there is a phase change of π upon reflection, so the central fringe is dark (destructive). The pattern is otherwise similar to YDSE. In Fresnel's biprism experiment, a biprism creates two virtual coherent sources from a single source. The effective slit separation d = 2a(μ-1)α, where a is the distance from source to biprism, μ is the refractive index, and α is the biprism half-angle. Fringe width β = λD/d as usual. These are common experimental setups tested in JEE.