Diffraction around Opaque Disk
Safety Warnings
Danger: Laser Safety - Class 3B
Purpose
Demonstrates Poisson's (Arago's) bright spot: light diffracting around an opaque disk produces a counterintuitive bright spot at the center of the geometric shadow. Provides compelling evidence for the wave nature of light through Fresnel diffraction.
Figure 1:
Three opaque dots of different sizes mounted in a finger clamp. The 1/32" dot (center) yields the best diffraction pattern.
Figure 2:
Fresnel diffraction pattern from an opaque dot, showing concentric rings and the Poisson bright spot at center.
Figure 3:
A well-aligned diffraction pattern with clear concentric fringes and a bright Poisson spot at center.
Figure 4:
Diffraction pattern from a different dot size, showing fewer rings and some asymmetry from alignment.
Figure 5:
Full optical rail setup: dot holder in finger clamp, green laser with reflection shield, and projection lens on three carriages.
Equipment
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