Diffraction by a Circular Aperture

This demonstration uses a 60mW He-Ne laser, which is a Class 3B laser device capable of causing permanent eye damage from direct or specular reflections. Never look directly into the beam or at specular reflections. Ensure the beam terminates on a diffuse surface (the projection screen). Remove reflective objects (watches, jewelry, glass) from the beam path. Consider posting laser warning signs and controlling room access during operation. The spatial filter creates an especially dangerous diverging beam near the laser source—maintain the safety shield and keep observers at least 1 meter from the laser output.

The laser power supply operates at line voltage. Inspect power cables for damage before use and ensure connections are secure. Do not operate the laser with the housing removed or if the power supply shows signs of damage.

Purpose

Students observe the Airy disk diffraction pattern — a bright central disk surrounded by concentric rings — produced when a laser beam passes through a circular aperture. The inverse relationship between aperture size and pattern size demonstrates wave optics and connects directly to the Rayleigh criterion for optical resolution.

Optical rail with wooden mounts, He-Ne laser with spatial filter at left, optical components along the rail, and metal reflection shield at right end

Figure 1: Complete apparatus showing the laser source with spatial filter at left, beam-expanding negative lens, and rotating stage holding various diffraction apertures. The setup produces Fraunhofer diffraction patterns projected onto a distant screen.

Danger: Class 3B Laser Hazard

Warning: Electrical Safety

Purpose

Safety Warnings

Danger: Class 3B Laser Hazard

Warning: Electrical Safety

Purpose