Diffraction is the spreading or bending of waves when they pass through a gap or move around an edge. It happens with water waves, sound waves, light waves, and other wave types. Diffraction matters because it explains why sound can be heard around corners, why light patterns form after passing through tiny slits, and why wave behavior depends strongly on size and scale.
In a slit diagram, straight incoming wavefronts become curved wavefronts after passing through the opening.
Key Facts
- Diffraction is strongest when the gap size is about the same as the wavelength.
- If gap width a is much larger than wavelength λ, diffraction is small and waves travel mostly straight.
- If gap width a is close to wavelength λ, waves spread out strongly after the gap.
- Wave speed relation: v = fλ, where v is wave speed, f is frequency, and λ is wavelength.
- For a single slit, minima occur approximately at a sin θ = mλ, where m = 1, 2, 3, ...
- Longer wavelengths diffract more than shorter wavelengths through the same opening.
Vocabulary
- Diffraction
- Diffraction is the spreading or bending of a wave as it passes through a gap or around an obstacle.
- Wavelength
- Wavelength is the distance from one point on a wave to the matching point on the next cycle, such as crest to crest.
- Wavefront
- A wavefront is a line or surface connecting points of a wave that are in the same phase.
- Slit
- A slit is a narrow opening that allows part of a wave to pass through a barrier.
- Single-slit pattern
- A single-slit pattern is the bright and dark distribution formed when light diffracts through one narrow opening.
Common Mistakes to Avoid
- Thinking diffraction only happens for light is wrong because all waves can diffract, including sound, water waves, and radio waves.
- Assuming a wider gap always gives more spreading is wrong because diffraction is strongest when the gap size is similar to the wavelength.
- Confusing diffraction with reflection is wrong because reflection is bouncing off a surface, while diffraction is spreading around an edge or through an opening.
- Ignoring wavelength when comparing waves is wrong because a long-wavelength sound wave can bend around a doorway much more than a short-wavelength light wave.
Practice Questions
- 1 A water wave has a speed of 2.0 m/s and a frequency of 0.50 Hz. Calculate its wavelength using v = fλ, then state whether a 4.0 m wide gap would cause strong or weak diffraction.
- 2 Light of wavelength 600 nm passes through a single slit of width 0.020 mm. Using a sin θ = λ for the first minimum, estimate θ in degrees for small angles.
- 3 Sound can often be heard around a corner, but visible light from a lamp usually does not bend noticeably around the same corner. Explain this using wavelength and diffraction.