Variable Rectangular Aperture - Physics Demonstrations - Variable Rectangular Aperture | Physics Undergraduate Labs

Use only Class 2 or Class 3R lasers (<5 mW) for classroom demonstrations. Never look directly into the laser beam or allow reflections to enter eyes. Position laser below eye level when possible. Ensure all students are positioned to view only the diffraction pattern on the screen, not the direct beam.

Purpose

Two independently adjustable slits (horizontal and vertical) form a variable rectangular aperture. Students observe in real time how narrowing one dimension widens the diffraction pattern in the perpendicular direction, demonstrating the inverse relationship between aperture size and diffraction spread.

$Green laser diffraction pattern showing a bright central maximum with secondary maxima arranged in a cross pattern along horizontal and vertical axes$

$Green laser diffraction pattern with wider horizontal spread and fewer vertical fringes, showing an asymmetric cross pattern$

Optical rail with green laser module on the left, two adjustable slit assemblies with control knobs in the center, and projection lens on the right

Figure 2: Rectangular aperture diffraction pattern with roughly equal slit widths, producing a symmetric cross pattern of secondary maxima in both directions.

Figure 3: Asymmetric aperture: widening one slit dimension compresses the diffraction pattern in that direction while keeping the perpendicular spread unchanged, demonstrating the inverse relationship between slit width and diffraction angle.

Figure 4: Complete setup: laser, two adjustable slits (horizontal and vertical) with independent control knobs, and projection lens on optical rail.

Safety Warnings

Warning: Laser Safety

Purpose