IGCSE Physics / Properties of waves
Reflection and refraction of waves revision notes
Edexcel International GCSE Physics (4PH1), chapter 10: Properties of waves, specification point 3.9. Free lesson.
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Reflection happens when waves bounce off a barrier. Refraction happens when waves enter a different medium and change speed, which changes their wavelength and can change their direction.
How waves reflect at a barrier
All waves can be reflected, including water, sound and light waves. In a ripple tank, each straight wavefront marks a crest. When the wavefronts meet a flat barrier, they bounce off. The normal is an imaginary line at right angles to the barrier, and both angles are measured from it. The angle of incidence, i, is the angle between the incoming wave's direction and the normal. The angle of reflection, r, is the angle between the reflected wave's direction and the normal. The rule for reflection is r = i. The reflected wavefronts stay the same distance apart because the speed…
Why waves refract
Refraction happens when a wave crosses into a different medium and changes speed. A medium is the material a wave travels through. Water waves travel more slowly in shallow water than in deep water, so the wavefronts become closer together. The wave's frequency stays the same because it is set by the source. Wave speed = frequency × wavelength, or v = f × λ. If speed decreases but frequency stays the same, the wavelength must decrease. If the wave arrives at the boundary at an angle, one end of each wavefront slows down first. This makes the wave change direction, turning towards…
Explain refraction in the right order
Start with the change that causes the others: the wave enters a new medium and its speed changes. Then use the fact that frequency stays fixed to explain the change in wavelength. If the wave arrives at an angle, explain why its direction changes too.
What happens to frequency when a wave slows down?
A wave slowing down does not mean its frequency decreases. The source sets the frequency, and the same number of waves reach the boundary each second as leave it. So when speed decreases, wavelength decreases while frequency stays the same.