Diffraction (definition)
● VERBATIM — examined word-for-wordDiffraction is the spreading of a wave into the region beyond an obstacle, or after it passes through an aperture, so that the wave does not travel only in straight lines. The wavelength, frequency and speed are unchanged; only the shape of the wavefronts changes.
What this actually means
Spreading, not bending in the refraction sense. Refraction changes speed and wavelength. Diffraction changes neither, which is exactly why examiners test the two together.
Explain it with Huygens if a why is asked. Every point on the wavefront in the aperture acts as a source of secondary wavelets; the envelope of those wavelets curves at the edges because the wavelets beyond the aperture have been removed.
The everyday example the syllabus likes is hearing someone around a corner without seeing them. Sound has a wavelength of about a metre, comparable with a doorway, so it spreads widely. Light has a wavelength of 5 × 10⁻⁷ m, far smaller than the doorway, so it barely spreads.
That comparison is the whole of the answer to why do we not see diffraction of light every day. It is a wavelength-to-aperture ratio argument, nothing else.
Diffraction happens at every aperture, including at each slit of a double slit and at the lens of every telescope. That is why the double-slit pattern sits inside a single-slit envelope and why telescopes have a resolution limit.
Saying the wavelength decreases after passing through a narrow gap. Diffraction leaves λ, f and v unchanged.
Prove it — watch it be true
- Open the ripple tank view and set the gap width to narrow.
- Confirm the wavefronts fan out into the region beyond the barrier instead of continuing straight.
- Check the wavelength readout is identical before and after the gap.