= Near-field diffraction laboratory experiment
A documented experiment uses curved illumination rather than the uniform <Fresnel slit propagation> model. <Laboratory Exercise No. 4: Diffraction, MIT 6.237, Fall 2024>, printed pp. 5–6, gives a helium–neon <laser> at $632.8\,\mathrm{nm}$, a microscope objective <optical lens>, a circular <aperture> and a screen 5–40 cm from the object. Preparation includes comparing central brightness in <Fresnel diffraction> and <Fraunhofer diffraction>. A 40× objective produces a nonplanar incident <wavefront>; the opening is placed less than 1 cm from it. With objective-to-screen distance fixed, students move the opening, sketch patterns, and record positions of consecutive central nulls to estimate aperture size. The <Fraunhofer diffraction> pattern supplies a regime comparison. A separate spherical-obstacle task observes a bright centre inside the shadow. The handout supplies procedures, rather than measured distances or profiles.<Laboratory Exercise No. 4: Diffraction, MIT 6.237, Fall 2024>[p. 1 introduction; p. 2 PL4.1(d); p. 5 §B and Fig. 3; p. 6 §§4.9–4.10: wavelength, apparatus, fixed-distance comparison and readout]{ref}
The circular-opening test probes cancellation of <complex amplitudes> at the centre while the source <wavefront> curvature changes. To interpret it, include that curvature in the aperture field $U_0$ and use a circular <aperture>, rather than substituting the plane-illuminated <Fresnel slit propagation> profile. Useful additional controls are to retain the same <laser> and opening, centre the illumination, repeat position readings and check background with the beam blocked. If a camera replaces the visual screen readout, avoid saturation and calibrate its position scale. Alignment, imperfect aperture edges, illumination variation and limited contrast near a null constrain the inferred size. A bright central obstacle spot illustrates why the sharp shadows of <geometrical optics> can fail; it does not imply that every <Fresnel diffraction> pattern has a bright centre.
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