Learning outcomes

4.1 Reflection

Reflection occurs when a wave meets a boundary and returns into the original medium. The frequency does not change because the source has not changed. In the same medium the speed is unchanged, so the wavelength is also unchanged. The direction changes according to the geometry of the boundary.

Plane water waves reflect from a straight barrier. Circular waves can reflect from curved surfaces. Sound reflects from hard surfaces to form echoes, while light reflects from mirrors and many ordinary objects. Smooth surfaces produce regular reflection; rough surfaces scatter waves in many directions.

4.2 Refraction

Refraction is a change in direction caused by a change in wave speed as a wave crosses a boundary. In a ripple tank, water waves move more slowly in shallow water than in deep water. If wavefronts meet the shallow region at an angle, one side slows first and the direction changes.

Frequency remains constant at the boundary. Since v = fλ, a reduction in speed causes a reduction in wavelength. Wavefronts become closer together in the slower region. A wave crossing normally may slow down and change wavelength without changing direction; this is still refraction even though no visible bending occurs.

4.3 Diffraction through a gap

Diffraction is the spreading of waves after passing through a gap or around an edge. It is most noticeable when the gap is similar in size to the wavelength. A narrow gap compared with the wavelength produces nearly semicircular wavefronts. A gap many wavelengths wide gives much less spreading, except near the edges.

Increasing wavelength while keeping the gap fixed increases diffraction. Decreasing the gap while keeping wavelength fixed also increases diffraction. The wave does not necessarily slow down during diffraction because it remains in the same medium.

4.4 Diffraction at an edge

When plane waves pass a single edge, the part of each wavefront beyond the edge spreads into the shadow region. Longer wavelengths bend more around the edge. This explains why low-frequency sound can be heard around corners more readily than high-frequency sound, since low frequency corresponds to long wavelength when speed is fixed.

Diffraction does not mean that waves turn sharply to follow the barrier. The wavefront curves outward from the edge, and intensity usually becomes lower as energy spreads over a wider region.

4.5 Comparing the three processes

Reflection: same medium, direction reverses or changes, speed and frequency unchanged. Refraction: boundary between regions where wave speed differs, frequency unchanged, wavelength changes and direction may change. Diffraction: spreading at a gap or edge, no necessary change of speed, frequency or wavelength.

A question may combine processes. For example, water waves can refract at a shallow region, reflect at a barrier and later diffract through an opening. Identify the event by describing the cause rather than relying only on the appearance of the wavefronts.

Worked examples

Practical focus

Examination guidance

Check your understanding

1. A wave enters a region where its speed halves. Its frequency stays constant. What happens to wavelength?
2. When is diffraction through a gap greatest?
3. Which process produces an echo?