An iris of radius is illuminated by an argon-ion laser at

PHYSICS · Multi-part · Fermi University Physics · Optics & Photonics · Interference and diffraction · Interference and diffraction

What an Instrument Can See, Ch Scalar diffraction theory, derived

Problem

An iris of radius

a = 0.50\,\mathrm{mm}

is illuminated by an argon-ion laser at

\lambda = 488\,\mathrm{nm}

. A screen is placed either

z_1 = 50\,\mathrm{mm}

z_2 = 5.0\,\mathrm{m}

beyond the iris. ((a)) Compute the Fresnel number

N_F = a^2/(\lambda z)

at each distance. ((b)) State which distance places the observation in the Fresnel regime and which in the Fraunhofer regime, giving the reason.