A Simple and Easy Tabletop Double-Slit Experiment to Directly Visualize Optical Intensity Distribution Pathways of Laser Light Beyond Diffracting Obstacles

Abstract

Naeem Ullah

Young's double-slit experiment, which represents the mystery of quantum physics and the diffraction that goes along with it, has long been used as a fundamental experiment to demonstrate how light is a wave by looking at interference patterns on a distant screen. In conventional diffraction experiments, which normally only show on a far field screen using photons, electrons, atoms, molecules, and other quantum systems. The evolution of the optical field in the Fresnel region is well-established theoretically, but there has not yet been much direct experimental observation of this intermediate field. In this work, we present the first direct experimental visualization of a structured Post-Obstacle Field (POF) immediately beyond a diffracting obstacle. The double-slit assembly consisted of two rectangular transmitting regions separated by a 50 μm diameter human hair acting as the central obstacle. A continuous-wave 632.8 nm He–Ne laser was used to illuminate the diffracting obstacle.

The Post obstacle Field (POF), an operational term that characterizes the optical intensity right after the diffraction barrier, is the name given to the experimentally observed intensity distribution referred to here as N dominant intensity pathways. POF exhibited two distinguishable but simultaneously occurring spatial structures: an approximately banded pattern and a radially organized, ring-like structure surrounding a structured central region. To independently verify that these structures represent a physical optical field rather than an imaging artifact, a transparent glass plate was positioned approximately 2 cm behind the obstacle. These observations suggest that spatial structure established in the near-field region may provide an experimentally observable precursor to the interference structure that develops during subsequent propagation. The experiment's simplicity of accessibility, affordability, and visual clarity make this work ideal for undergraduate labs and serve as a useful link between theoretical explanations of diffraction and experimental observation.

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