Trap: diffraction does not change the wavelength
When a wave diffracts at a gap or an edge, its speed, frequency and wavelength are all unchanged. Only the shape of the wavefronts and the amplitude change. The same is true for superposition and interference: the component waves keep their own λ, f and v throughout.
What this actually means
In ripple-tank sketches, the wavefronts after the gap must be drawn with exactly the same spacing as before. Squeezing them up is a guaranteed lost mark.
The thing that changes is direction of travel and amplitude. The wave spreads, and because the energy is spread wider the amplitude falls.
Compare with refraction, which is the process that does change speed and wavelength (frequency stays fixed). Examiners set diffraction and refraction side by side precisely to catch the confusion.
Same logic in a stationary wave: the component progressive waves keep their original wavelength, which is why node spacing is λ/2 of that original wavelength.
If you are ever asked what happens to the frequency in any of these processes, the answer is always that it is unchanged, because frequency is set by the source.
Drawing shorter wavelengths after a narrow gap, or writing that the wave slows down when it diffracts.
Prove it — watch it be true
- In the ripple tank view, read the wavefront spacing before the barrier.
- Narrow the gap until the spreading is dramatic, then read the spacing after the barrier.
- Confirm the two spacings are identical while the wavefront curvature and amplitude have changed.