A student investigates wave reflection using a ripple tank. She places a straight metal barrier at an angle in the tank and sends plane waves towards it. Sh...
Question 1 Report
A student investigates wave reflection using a ripple tank. She places a straight metal barrier at an angle in the tank and sends plane waves towards it. She records the angle of incidence and the angle of reflection for each trial. Table 19.1 shows her results.
| Trial | Angle of incidence i / ° | Angle of reflection r / ° |
|---|---|---|
| 1 | 15 | 15 |
| 2 | 30 | 30 |
| 3 | 45 | 44 |
| 4 | 60 | 60 |
| 5 | 75 | 76 |
(a) State the law of reflection that these results support. [1]
(b) State the property of the waves that does not change after reflection. Choose two from the list: speed, direction, wavelength, amplitude. [2]
(c) Explain what is meant by the term normal as used in the law of reflection. [1]
(d) In trial 3, the measured angle of reflection is 44° instead of 45°. Suggest one reason for this discrepancy. [1]
(e) The student then replaces the flat barrier with a curved (convex) barrier. Describe how the reflected wavefronts differ from those produced by the flat barrier. [1]
Answer Details
Marking Scheme
- (a) The angle of incidence equals the angle of reflection [1]
- (b) Speed [1]; wavelength [1]
- (c) The normal is a line drawn at right angles / perpendicular to the reflecting surface at the point where the wave hits it [1]
- (d) Measurement error / difficulty in determining the exact wavefront positions / parallax when reading the angle [1]
- (e) The reflected wavefronts spread outward / diverge instead of remaining parallel plane waves [1]
Explanation
(a) The law of reflection states that the angle of incidence is equal to the angle of reflection. Looking at the data, for each trial the two angles are the same (within experimental error), which confirms this law.
(b) When a wave reflects, its speed and wavelength do not change because it remains in the same medium. The direction changes (that is the whole point of reflection), and the amplitude may decrease slightly due to energy absorption, but speed and wavelength are preserved.
(c) The normal is an imaginary line drawn perpendicular (at 90°) to the reflecting surface at the exact point where the incoming wave meets the surface. Both the angle of incidence and the angle of reflection are measured between the wave ray and this normal, not from the surface itself.
(d) In trial 3, the measured angle of reflection is 44° rather than the expected 45°. This 1° discrepancy is within normal experimental uncertainty. It could arise from difficulty in judging the exact position of the wavefronts in the ripple tank, parallax error when reading the protractor, or slight imperfections in the barrier alignment.
(e) A flat barrier produces plane reflected wavefronts because each part of the incoming plane wave reflects at the same angle. A convex (outward-curving) barrier reflects different parts of the wavefront at different angles, causing the reflected wavefronts to diverge (spread outward) rather than remaining parallel.
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