Diffraction from a Wire
Safety Warnings
Danger: Class 3B Laser Hazard
Warning: Electrical Safety
Purpose
A laser beam diffracts around a thin wire, producing a fringe pattern equivalent to single-slit diffraction of the same width (Babinet's principle). Students see that light bends around opaque obstacles and that complementary diffracting objects produce identical diffraction patterns.
Figure 1:
Diffraction pattern from a thin wire. The dark central line is the wire's geometric shadow; diffracted light fills in on both sides with visible fringes from single-slit-like diffraction.
Figure 2:
Wire diffraction pattern showing the characteristic fringe spacing. Minima occur at angles satisfying $a\sin\theta = m\lambda$ where $a$ is the wire diameter.
Figure 3:
Wire diffraction pattern with multiple fringes visible. The fringe spacing can be used to calculate the wire diameter.
Figure 4:
Complete optical setup: laser with spatial filter, rotating stage with wire/diffraction objects, projection lens, and negative (diverging) lens for expanding the pattern onto the screen.