Explain reflection of waves at a plane surface
Trace a returning wave pattern and keep the normal separate from the reflecting surface.
Section P3.1 · General properties of waves
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-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. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Recognise a propagation direction normal to a simple wavefront.
- Distinguish the same medium from a changed medium.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which reference is used for an angle of incidence?
What you will learn
- Describe wave reflection at a plane surface.
- Compare incident and reflected directions and matching wavefronts.
The wave returns into the original region
Reflection occurs when a wave meets a boundary and a returning disturbance travels back into the original region. In the simple plane-surface model, an incident direction approaches the surface and a reflected direction leaves it. The normal is perpendicular to the surface at the point of incidence; it is a reference line, not another reflecting wall.
Angles of incidence and reflection are measured from that normal. In the ideal plane reflection shown, equal angles describe the two directions. A 35° incident angle to the normal gives a 35° reflected angle to the normal, or 55° to the surface. The drawn arrows show propagation directions rather than the material particles travelling into and away from a wall.
Compare the original and returned pattern
For a stationary source and stationary boundary in the same uniform medium, the reflected wave has the same frequency and speed as the incident wave, so its wavelength is unchanged. Direction changes. Those conditions matter: do not assume every moving boundary or changed medium has the same simple account.
The boundary can transfer some energy to other processes, so unchanged frequency does not guarantee equal incident and reflected amplitude or perfect reflection. Wavefronts are perpendicular to the model's rays, and neighbouring fronts retain the same wavelength spacing in this same-medium reflection. The sketch supplies geometry, not measured reflected energy fractions.
Sources for this lesson
Pause and explain
At 35° incidence to the normal, what is the ideal reflected angle?
Worked example
A wave reaches a stationary plane reflector at 35° to its normal in a uniform medium. State reflected direction angle and wavelength change.
Show the worked solution
- Measure angles from the normal.
- The reflected angle is 35° to the normal, hence 55° to the surface.
- Under the stated stationary same-medium conditions, wavelength is unchanged.
Answer 35° to the normal; 55° to the surface; wavelength unchanged.
Try a topic test
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Test this topic →From guided practice to a new situation
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 a wave reflecting at a stationary plane boundary in the same uniform medium.
Give me a hint
Track the approaching and returning directions.
Compare my reasoning
- An incident disturbance approaches the boundary.
- A reflected disturbance returns into the original region.
- The ideal directions make equal angles to the normal.
A returning pattern with equal normal-referenced angles in the ideal plane model.
Look for these in your work
- Distinguish both directions.
- Name the normal reference.
A wave arrives at 20° to the surface. Find incident and reflected angles to the normal.
Give me a hint
Convert the surface angle before applying reflection.
Compare my reasoning
- Incidence to normal = 90 − 20 = 70°.
- Reflection to normal is also 70°.
- Both directions make 20° to the surface.
Incident and reflected angles are both 70° to the normal.
Look for these in your work
- Use complementary angles.
- Keep the reference stated.
An incident and reflected wave have equal wavelength in the stated same medium. Does that prove equal amplitudes?
Give me a hint
Wavelength and amplitude describe different features.
Compare my reasoning
- Wavelength describes a spatial repeat.
- Amplitude describes local displacement magnitude.
- Boundary energy transfers can reduce the returned amplitude without changing the repeat length.
No; equal wavelength does not establish perfect reflection or equal amplitude.
Look for these in your work
- Separate the quantities.
- Retain the boundary-energy limit.
Common mistakes
- Measure angles from the normal.
- Returning pattern motion is not a full material journey.
- Equal frequency does not guarantee equal amplitude.
What can you explain now?
An incident angle is 20° to a reflecting surface. What is its angle to the normal?
Compare with the explanation
70°.
The normal is perpendicular to the surface, so the two acute angles sum to 90°.
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Make it stick
Describe plane reflection and separate normal-referenced direction changes from wavelength and amplitude claims.
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.