Young’s double slit experiment & Spatial coherence of light Ivana Hamarová

Monochromatic plane wave propagating along axis z Electrical field Phase j amplitude l l j = 2p

Monochromatic plane wave Electrical field Phase j amplitude l l j = 2p crests troughs

wavefronts Plane wave Spherical wave

Young’s double slit experiment Path difference Phase difference x z Constructive interference + Destructive interference +

Young’s double slit experiment z D >> a, a >> l Dj.. phase difference d… path difference a….distance between slits D…distance between slits and observation plane Period of interference pattern

Effect of slit width on the interference pattern light block

Monochromatic plane wave Electrical field Phase j amplitude l l j = 2p

Monochromatic plane wave Electric field amplitude Phase j E j = 0 …t1, E(z0,t1) j = p/2 …t2, E(z0,t2) j= p …t3, E(z0,t3) z0 z

Phase at two points x1, x2 x j1 = 0 …t1 j 1= p/2 …t2 x1 j 1= p …t3 z phase difference between two points j 2 - j 1 = Dj = 0, 0, 0 is constant in time = spatially coherent light

Phase at two points x1, x2 Disturbance (x) (x,t) x x1 z x2

Phase at two points x1, x2 t1 t2 Disturbance (x,t1) x x1 z x2 phase difference between two points j 2 - j 1 = Dj ≠ konst is not constant in time =spatially incoherent light

Complex degree of spatial coherence g(Δx) ..describes spatial coherence of light ..a measure of the degree of spatial coherence ..function of distance Dx=x2-x1

Spatial coherence of light Δx x1 x2 1 2

Complex degree of spatial coherence g(Δx) for all points of interest => Fully spatially coherent light for x1 ≠ x2 => Fully spatially incoherent light

Complex degree of spatial coherence g(Δx) For experimental setup (far field) y h rectangular aperture x x2 Disturbance z Δx x1 R h.. aperture width R...distance between rectangular aperture and the points x1 and x2 coherence distance First minimum of function g (x1,x2)

Effect of the aperture width on the spatial coherence ac coherence distance ac increases as aperture width h decreases ac

Effect of the aperture width on the spatial coherence ac a h h a..distance between S1 and S2 a = ac => g(a) =0 a < ac => g(a) = 0.55

Visibility of interference fringes (intensity modulation)= ac a |g(Δx)| I(x) a << ac visibility = 1 x I(x) a < ac partial visibility x I(x) a=ac visibility = 0 x

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