Describe diffraction through a narrow gap
Recognise spreading beyond an opening without treating the wave as material flowing through a hole.
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
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.
Sources for this lesson
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.