Section 1 of 4Refraction at a Boundary
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A wave moving obliquely into shallower water slows down, so its wavelength decreases.
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One side of the wavefront enters the shallow region first and slows, while the other side is still moving quickly.
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This difference in speed turns the wavefronts towards the normal.
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The frequency stays the same, because the source sets how many waves are produced each second.
Section 2 of 4Refraction Rules
Slower speed
- When a wave slows on entering a denser medium, it bends towards the normal.
Faster speed
- When a wave speeds up on entering a less dense medium, it bends away from the normal.
Perpendicular entry
- A wave hitting a boundary at 90° is not bent, because the whole wavefront changes speed at the same time.
Water depth
- Water waves travel faster in deeper water, so they refract whenever the depth of the water changes.
Section 3 of 4Diffraction
Diffraction
Definition
- Diffraction is the spreading out of a wave as it passes through a gap or past the edge of an obstacle.
Gap size
- Diffraction is greatest when the gap width is about the same as the wavelength, and it decreases as the gap widens.
Edges
- Waves also curve round the edge of a barrier, so the shadow behind the barrier is not sharply defined.
Sound
- Sound diffracts through doorways and around objects, which is why you can hear someone round a corner.
Section 4 of 4Exam Checklist & Tips
3.1.5 – Key Points for the Exam
- Refraction The change in direction of a wave caused by a change in its speed at a boundary.
- Diffraction The spreading of a wave as it passes a gap or the edge of an obstacle.
- Normal A construction line drawn perpendicular to a boundary.
- Wavefront A line joining points of a wave that are at the same stage of vibration.
- Gap An opening through which a wave can pass, with a width that can be compared with its wavelength.
Common Mistakes
- Saying the frequency of a wave changes during refraction.
- Saying waves bend away from the normal when they slow down, when they actually bend towards it.
- Saying diffraction is greatest when the gap is much wider than the wavelength.
- Confusing diffraction, which is spreading, with reflection, which is bouncing back.