13 Jul 2026

Thickness of a Thin Wire by Laser Diffraction

practical pg-i laser diffraction thin-wire

Aim

To determine the diameter of a thin wire using the diffraction pattern produced by a laser.

Apparatus

Laser, thin wire, screen, metre scale, wire holder, and optical bench.

Experimental arrangement

The wire is placed in the laser beam and the positions of successive diffraction minima are measured on the screen.

Theory

By Babinet’s principle, the diffraction pattern of a thin opaque wire has the same positions of minima as a slit of width equal to the wire diameter $a$. For a screen distance $D$ and distance $x_n$ of the $n$th minimum from the centre,

\[a\frac{x_n}{D}=n\lambda,\]

for small angles. Hence $a=n\lambda D/x_n$.

Observations

Minimum order $n$	Distance from centre $x_n$ (cm)	Screen distance $D$ (cm)
1	2.60	100
2	5.20	100
3	7.80	100

For $\lambda=650$ nm, the diameter is approximately $0.025$ mm.

Result

The diameter of the wire is approximately $\boxed{0.025\,\text{mm}}$.

Viva Questions

1. Why can Babinet’s principle be used? The complementary wire and slit have the same diffraction intensity away from the geometrical image.
2. Why is a laser preferred? It is monochromatic and highly directional.
3. What causes the dark bands? Destructive interference of secondary wavelets.