What is the distance of the third dark fringe from the central maximum in a double-slit experiment if \( \lambda = 600 \
**Incoherent sources** intensity adds I = I₁+I₂, no interference pattern because phase random, two independent sources cannot produce stable interference because phase difference fluctuates rapidly, coherent sources required with constant phase, laser coherent, visibility of fringes requires coherence, degree of coherence determines contrast. Dark fringe position x_n = ((n + (1/2)) λ D/d) . For the third dark fringe, n = 2 . λ = 6.0 × 10⁻⁷ m , d = 4.0 × 10⁻⁴ m , D = 2.0 m . x₂ = ((2 + (1/2)) × 6.0 × 10⁻⁷ × 2.0/4.0 × 10⁻⁴) = (2.5 × 1.2 × 10⁻⁶/4.0 × 10⁻⁴) = 7.5
Ref: NCERT > Physics Book > Wave Optics > Optical Phenomena and Applications