8.2 Diffraction
- Syllabus
- 9702–2028–2029
- Topic
- 8.2
- Level
- AS
Learning objectives
8.2.1The meaning of the term diffraction• explain the meaning of the term diffraction8.2.2Experiments that demonstrate diffraction including the qualitative effect of• show an understanding of experiments that demonstrate diffraction including the qualitative effect of the gap width relative to the wavelength of the wave• For example diffraction of water waves in a ripple tank
Diffraction spreads a wave through an aperture or around an edge
Diffraction is the spreading of a wave after it passes through an aperture or past an obstacle/edge, including spreading into the geometrical shadow.
The effect is most noticeable when aperture width or obstacle size a is comparable to wavelength λ. If a≫λ, most wavefronts continue nearly straight and only the edges spread strongly.
Plane wavefronts through a very narrow gap emerge approximately semicircular; a wider gap leaves a flatter central region with curved edges. Around an obstacle, waves bend into the region geometric rays would not reach.
If the wave remains in the same medium, diffraction does not change its frequency, speed or wavelength. It redistributes direction and therefore amplitude/intensity across positions.
Long-wavelength sound/radio can reach around doors, buildings or hills more readily than visible light because the relevant obstacles are closer to those wavelengths.
Reflection is bouncing at a boundary; refraction is direction/speed change on entering another medium; interference is superposition. Diffraction can help waves overlap, but it is the spreading at the aperture or edge.
Compare diffraction patterns by changing one side of the a/λ ratio
Choose one wave source and geometry; vary only aperture width a or wavelength λ, then compare angular spread, wavefront curvature or central-maximum width at a fixed observation distance.
smallera/λ→greaterspreadinglargeraatfixedλ→lessspreadinglargerλatfixeda→greaterspreading
Ripple tank: use straight incident wavefronts and adjustable gap; strobe/freeze the image if needed. A gap near one wavelength gives strongly curved emerging wavefronts while spacing between successive fronts remains λ.
Laser and single slit: keep slit-screen distance fixed. A narrower slit or longer wavelength gives a wider central maximum; observe safely on a screen and never look into the beam.
Sound/microwave: move a detector behind an aperture/obstacle and map received amplitude. Longer wavelength spreads farther into the geometrical shadow for the same opening.
A broader pattern does not mean the wave travelled faster or its wavelength changed after the gap. It shows that a/λ is smaller; compare like observations with source, medium and distances controlled.