explain the meaning of the term diffraction

Published by Patrick Mutisya · 8 days ago

Diffraction – Cambridge A-Level Physics 9702

Diffraction

Objective

Explain the meaning of the term diffraction and its relevance to wave optics.

Definition

Diffraction is the bending and spreading of a wave when it encounters an obstacle or a slit whose dimensions are comparable to the wavelength of the wave. It is a direct consequence of the wave nature of light (or any other wave) and Huygens’ principle, which states that every point on a wavefront acts as a source of secondary spherical wavelets.

Key Features

  • Occurs when the size of an aperture or obstacle is of the order of the wavelength \$\\lambda\$.

  • The wavefront emerging from the aperture is no longer planar; it spreads out in space.

  • Produces characteristic intensity patterns (fringes) on a screen.

  • More pronounced for longer wavelengths and smaller apertures.

Single‑Slit Diffraction

When a monochromatic plane wave of wavelength \$\\lambda\$ passes through a narrow slit of width \$a\$, the condition for minima in the diffraction pattern is given by

\$a \sin\\theta = m\\lambda \\quad (m = \\pm1, \\pm2, \\dots)\$

where \$\\theta\$ is the angle measured from the central axis and \$m\$ is the order of the minimum.

The intensity distribution as a function of angle \$\\theta\$ is

\$\$I(\\theta) = I_0 \\left(\\frac{\\sin\\beta}{\\beta}\\right)^2, \\qquad

\\beta = \\frac{\\pi a \\sin\\theta}{\\lambda}\$\$

\$I_0\$ is the maximum intensity at \$\\theta = 0\$.

Comparison with Interference

AspectDiffractionInterference
Origin of patternWavefront bending around a single aperture or obstacleSuperposition of waves from two or more coherent sources
Typical conditionFeature size \$\\sim \\lambda\$Path‑difference \$\\approx n\\lambda\$ (with \$n\$ integer)
PatternBroad central maximum with diminishing side fringesEqually spaced bright and dark fringes
Dependence on wavelengthMore pronounced for longer \$\\lambda\$Fringe spacing \$\\propto \\lambda\$

Everyday Examples

  1. Faint shadows observed behind a hair illuminated by a laser pointer.

  2. The spreading of sound around a doorway.

  3. The pattern of light on the edge of a razor blade in a dark room.

Suggested diagram: Ray diagram showing a plane wave incident on a narrow slit and the resulting diffraction pattern on a distant screen, with angles \$\\theta\$ marked for minima.

Why Diffraction Matters in A‑Level Physics

Understanding diffraction is essential for:

  • Interpreting experimental results from slit experiments.

  • Designing optical instruments (e.g., telescopes, microscopes) where resolution is limited by diffraction.

  • Explaining phenomena such as the resolving power of a spectrometer and the Airy disc.

Summary

Diffraction is the bending and spreading of waves when they encounter obstacles or apertures comparable in size to their wavelength. It leads to characteristic intensity patterns that can be predicted using Huygens’ principle and the mathematical relations shown above. Mastery of diffraction concepts enables students to analyse a wide range of optical phenomena and to appreciate the wave nature of light.