Diffraction: spreads, nothing else changes
Diffraction is the spreading of a wave as it passes through a gap or around an obstacle — both elements are needed for the mark. It does not change the wave's speed, frequency or wavelength (same medium, same source; stays fixed). What changes is the direction at the edges and the energy distribution — the wavefronts curve round and the beam spreads out.
Most diffraction when gap ≈ wavelength
Diffraction depends on the ratio of wavelength to gap width. Maximum when gap — wavefronts emerge as semicircles. If , only the edges bend and a geometric shadow forms. Rules: smaller gap → more diffraction (fixed ); longer wavelength → more diffraction (fixed gap). E.g. radio ( ~metres) diffracts round buildings; visible light ( ~500 nm) barely diffracts through a 1 m doorway.
Radio diffracts, light barely does
Diffraction varies hugely across the EM spectrum because gap/wavelength varies. FM radio ( m) through a ~1 m doorway gives → strong diffraction. Visible light ( nm) through the same doorway gives → negligible. A diffraction grating (lines ~500 nm apart) has gap , spreading light into colours. When , diffraction is maximum.
Drawn from real examiner reports.
Thinking diffraction changes speed or f
Do not write that diffraction slows the wave or changes its frequency — that confuses it with refraction. Through a gap in the same medium the wave spreads, but its speed, frequency and wavelength are all unchanged; only the direction at the edges changes. If a question asks "what stays the same?", state all three explicitly — often a mark itself.
Thinking a bigger gap diffracts more
The intuitive "bigger gap lets more through, so more diffraction" is backwards. Diffraction is maximum when ; a larger gap (relative to ) gives less spreading. To increase diffraction, narrow the gap towards , or use a longer wavelength. Answer with the gap/wavelength ratio, never the absolute gap size.
Vague "waves bend" definition
"Diffraction is when waves bend" is too vague for full marks. Two elements are needed: (1) waves spread out, and (2) as they pass through a gap or around an obstacle. A complete answer: "the spreading of waves as they pass through a gap or around the edge of an obstacle". Naming only the gap, or only "bending", loses a mark.
Frequency left in kHz or MHz
When finding from , convert the frequency to hertz first: 40 kHz = 40 000 Hz; 200 MHz = Hz. In June 2022 Paper 1P Q3 many candidates left the frequency in kHz, giving a wavelength wrong by a factor of 1000 — the examiner called it the most common reason for losing the mark.
June 2022 Paper 1P Q3(b)(ii): failure to convert kHz to Hz gave a wavelength wrong by a factor of 1000 — the most common reason for not gaining the third mark.
Wavefronts squeezed after the gap
On a diffraction wavefront diagram, keep the spacing between wavefronts (the wavelength) the same before and after the gap. Drawing them closer together afterwards implies the wave slowed and the wavelength shrank — that is refraction, not diffraction. Only the shape changes: flat wavefronts become curved.
Inverting λ = v/f
Rearranging , watch the direction: and . Dividing by (e.g. ) inverts it and gives a value in m⁻¹, not metres. Do a unit check: m/s ÷ Hz = m; m/s ÷ m = Hz.
(general exam technique)
Define, compare λ to gap, state conserved
Scaffold: (1) define diffraction with both elements (spread + gap/obstacle); (2) compare to the gap — means maximum spreading, little; (3) if asked what changes, state speed, frequency and wavelength are unchanged.
How to increase diffraction
For "how do you increase diffraction?", answer using the ratio: make the gap narrower (closer to ) or use a longer wavelength. Never say "a wider gap" — that reduces it. State the gap/wavelength comparison, not the absolute gap size.
Convert frequency to Hz first
Before using , convert the frequency to hertz: kHz , MHz , GHz . Show the converted value in your working. Then a wavelength answer in metres is trustworthy; leaving it in kHz gives an answer wrong by ×1000.
Draw wavefronts curved, same spacing
Drawing a diffraction diagram: plane wavefronts approach with arrows; mark the gap width ; beyond the gap draw curved wavefronts with the same spacing . For they are near-semicircular. Label and — unlabelled loses marks.
Diffraction itself does not have a unique calculation formula — it is a qualitative wave behaviour. However, it is always linked to the wave equation and the wavelength-to-gap comparison:
where = wave speed (m/s), = frequency (Hz), = wavelength (m).
The condition for maximum diffraction:
where = gap width (m) and = wavelength (m).
Frequency-period relationship:
where = time period (s).
These formulas are given on the formula sheet — the key skill is recognising when diffraction context requires wavelength calculation, and then converting units correctly.
Define diffraction.
Water waves in a ripple tank travel at 0.30 m/s and have a frequency of 5.0 Hz. Calculate the wavelength of the water waves.
