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#dark fringes

2 public questions tagged with this topic.

Polarization and Malus Law

What causes the intensity of light to remain conserved during interference despite the presence of dark fringes?

**Two polaroids** with pass-axes perpendicular intensity zero because second blocks component, with 45° intensity I₀/4 if initial after first I₀, if initial unpolarized I₀, after first I₀/2 then after second at 45° I₀/4. Three polaroids first and third crossed 90°, middle at 45° maximum transmission because middle rotates polarization, I after first I₀/2, after middle at 45° I₀/4, after third at 45° to middle I₀/4×cos²45°= I₀/8, non-zero, maximum when middle 45°. Energy is redistributed from dark to bright fringes through interference, conserving total energy as the sum of intensities balances

Ref: NCERT > Physics Book > Wave Optics > Polarization and Malus Law

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Wavefront and Huygens Principle

What is the condition for destructive interference in a double-slit experiment?

**Wavefront types** point source spherical, distant point source plane, after convex lens plane wave focuses to point because lens adds phase delay proportional to thickness, converging spherical wavefront, after concave mirror plane wave becomes spherical converging to focus, illustrating Huygens construction. Destructive interference occurs when the path difference is an odd multiple of half the wavelength, i.e., Δ = (n + (1/2))λ . Using Δ = d sinθ, y = n λ D/d, a sinθ = n λ, I = I₀ cos²θ, n = c/v, sinC = 1/n, λ' = λ/n and A = 2a cos(φ/2), calculation gives Path difference = (n + (1/2))λ, il

Ref: NCERT > Physics Book > Wave Optics > Wavefront and Huygens Principle

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