Describe diffraction through a narrow gap
Recognise spreading beyond an opening without treating the wave as material flowing through a hole.
Exam scope and assessment notes
Core and Extended · Physics 0972, 2026–2028, section 3.1. Core covers energy transfer, rope/spring/water-wave observations, wave features, speed, transverse and longitudinal models, reflection, refraction and ripple-tank diffraction. Extended compares wavelength effects at gaps and edges, including gap size. Written work is self-checked; human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0972 has the same subject content as IGCSE 0625, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Recognise matching wavefronts.
- Distinguish pattern direction and local medium motion.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which observation supports narrow-gap diffraction?
What you will learn
- Describe wave diffraction through a narrow gap.
- Separate spreading from changed frequency, absorption or a material jet.
The transmitted pattern spreads beyond the gap
Diffraction is the spreading of waves through an opening or around an obstacle into regions beyond a simple straight-path prediction. In the suitable narrow-gap ripple-tank model, incident straight wavefronts reach the opening and spreading curved fronts emerge beyond it. Draw the blocked barrier parts and the actual opening; a full solid barrier is not a gap experiment.
This is a wave-pattern effect, not proof of a one-way water jet carrying the same material parcels through the whole pattern. The field beyond the opening need not remain confined to the incident straight-line width. A single straight ray does not display the spread of many propagation directions beyond the gap.
Keep unchanged-region properties separate from shape
With a stationary source and the same uniform propagation conditions on both sides, the transmitted frequency and speed are unchanged, so wavelength is unchanged. The wavefront shape and spread of directions change. A barrier can reduce transmitted energy or amplitude, but that does not by itself require a new cycle rate.
The supplied curved fronts show qualitative spreading rather than measured amplitudes, exact intensity or a calculated diffraction angle. Core learners describe the narrow-gap effect. The separately labelled Extended lessons compare wavelength and gap size systematically; no universal numeric spreading angle follows from this sketch.
Pause and explain
Same-medium diffraction at a stationary opening changes which described feature?
Worked example
Straight water wavefronts approach a narrow opening in a stationary barrier. The water conditions are the same on both sides. Describe the emerging pattern and frequency.
Show the worked solution
- The transmitted fronts spread beyond the opening.
- Curved fronts represent diffraction in the simplified model.
- The stationary source frequency is unchanged.
Answer Spreading curved fronts beyond the gap; frequency unchanged under the stated conditions.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe straight fronts reaching a narrow gap and the pattern beyond it.
Give me a hint
Compare incident shape with the emerging spread.
Compare my reasoning
- Incident fronts approach the opening.
- Barrier parts block other direct paths.
- Curved transmitted fronts spread beyond the gap.
The emerging pattern spreads as curved fronts beyond the opening.
Look for these in your work
- Show the gap.
- Describe sideways spreading.
For a stationary source and unchanged water conditions, state what happens to f, v and λ after a gap.
Give me a hint
Use the unchanged source and medium conditions with v = fλ.
Compare my reasoning
- Frequency retains the source rate.
- Speed retains the same uniform-medium value.
- Wavelength therefore remains unchanged despite spreading.
Frequency, speed and wavelength unchanged; pattern spreads.
Look for these in your work
- State the conditions.
- Separate repeat length from shape.
A sketch shows stronger curvature beyond a gap but no amplitude scale. Can it determine transmitted energy percentage?
Give me a hint
Geometry alone does not supply an energy calibration.
Compare my reasoning
- The fronts show pattern geometry.
- Amplitude and relevant energy measurements are not supplied.
- No numeric transmission percentage follows.
No; the sketch does not establish a transmitted energy percentage.
Look for these in your work
- Identify missing measurements.
- Avoid inferring intensity from curvature.
Common mistakes
- Draw an actual opening between barrier parts.
- Pattern spreading is not a tagged-water jet.
- A sketch does not give exact intensity or diffraction angle.
What can you explain now?
Does a curved transmitted front alone prove that the source frequency changed?
Compare with the explanation
No.
Spreading changes the pattern's directions; the stationary source can retain the same cycle rate.
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Make it stick
Describe a narrow-gap diffraction pattern and keep geometry separate from source frequency and transmission amount.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.