Practice questions · Reflection, refraction, diffraction

Analyse the situation. Then explain.

The questions students fear most on this topic are the ones that say "explain what happens and why". Eight original questions, four of them analysis-and-explain. The worked solutions show you a reusable method, not just an answer.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

The method for "explain what happens" questions

When a wave crosses a boundary, check four things in this order.

Most analysis questions on this topic are secretly the same question. A wave meets a boundary or an obstacle, and you must say what changes. Work through these four, every time, in order:

Speed. Did the medium change? If yes, speed changes. This is the cause of everything else.
Wavelength. It follows the speed, because v = fλ and frequency is fixed. Slower means shorter.
Frequency. Set by the source. It never changes at a boundary. State this explicitly.
Direction. Only bends if the wave meets the boundary at an angle. Head-on, no bend.

If you write your answer in that order, you will almost never miss a mark.

[3 marks]

A plane water wave is reflected from a straight barrier in a ripple tank.

(a) State the relationship between the angle of incidence and the angle of reflection. [1] (b) State what happens to the speed, frequency and wavelength of the wave during reflection. [2]

(a) The angle of incidence equals the angle of reflection (both measured from the normal). ✓

(b) Speed, frequency and wavelength all stay the same. Only the direction changes. ✓✓

Reflection is the "nothing changes except direction" behaviour. The medium has not changed, so there is no reason for speed or wavelength to change.

Analysis

[4 marks]

In a ripple tank, plane water waves travel from a region of deep water into a region of shallow water. The boundary between the regions is at an angle to the wavefronts.

Analyse what happens to the speed, wavelength, frequency and direction of the waves as they cross into the shallow water. Give a reason for each.

Work the four properties in order:

Speed decreases. Water waves travel more slowly in shallower water.
Wavelength decreases. Because v = fλ and frequency is fixed, a smaller v means a smaller λ.
Frequency stays the same. It is set by the source (the vibrating bar), not by the medium.
Direction bends toward the normal. Because the waves meet the boundary at an angle, one end of each wavefront slows before the other, twisting the wave toward the normal.

Four marks, four properties, each with a reason. This is the template for every "wave crosses a boundary" question. Notice frequency is the one that does NOT change, and that catches students out.

Analysis

[4 marks]

A student is confused. They write: "When a wave slows down as it enters a new medium, its frequency must drop, because it is moving more slowly."

Explain why the student is wrong. In your answer, make clear what actually happens to the frequency and the wavelength, and why.

Frequency is set by the source, which has not changed. The same number of wavefronts arrive at the boundary each second as leave it. So frequency stays constant.
If frequency stayed the same but wavefronts piled up at the boundary, that would be impossible. So instead the wavelength shrinks.
It is the wavelength that absorbs the change in speed, not the frequency: v = fλ, with f fixed, so a smaller v forces a smaller λ.

The key mental image: imagine standing at the boundary counting wavefronts per second. The count cannot change, or waves would appear or vanish. So f is fixed; λ does the changing.

Diagram

[4 marks]

The diagram shows plane water wavefronts travelling straight downward and crossing a boundary between two regions of a ripple tank. The wave meets the boundary head-on (the wavefronts are parallel to the boundary).

Wavefronts crossing a boundary head-on. Spacing changes from Region X to Region Y.

(a) Name the behaviour shown in the diagram. [1] (b) State which region, X or Y, is the shallower water. Explain your answer. [2] (c) The wave does not change direction here. Explain why. [1]

(a) Refraction. ✓

(b) Region Y is shallower. The wavefronts are closer together there, so the wavelength is shorter, which means the wave has slowed down, and water waves are slower in shallower water. ✓✓

(c) The wave meets the boundary head-on (wavefronts parallel to the boundary), so both ends of each wavefront slow at the same instant. With no twisting, there is no change in direction. ✓

Closer wavefronts = shorter wavelength = slower wave = shallower water. Follow that chain and part (b) is automatic.

Analysis

[4 marks]

Two people stand on opposite sides of an open doorway, around a corner from each other, so that they cannot see one another. They find they can hear each other talking clearly, but cannot see each other at all.

Analyse this situation using the idea of diffraction. Explain why the sound reaches around the corner but the light does not.

Both sound and light diffract (spread out) when they pass through the doorway.
The amount of diffraction depends on how the wavelength compares with the width of the gap (the doorway).
Sound has a long wavelength, comparable to the size of the doorway, so it diffracts strongly and spreads around the corner to the listener.
Light has an extremely short wavelength, far smaller than the doorway, so it barely diffracts and continues almost in a straight line. It does not bend around the corner, so they cannot see each other.

Structure: state both diffract, state the rule (wavelength vs gap), then apply it to each wave. The marks are in the comparison, not just in naming diffraction.

Diagram

[3 marks]

The diagram shows plane water waves in a ripple tank approaching a gap in a barrier, and what they look like after passing through.

Waves passing through a gap in a barrier.

(a) Name the behaviour shown. [1] (b) State what happens to the wavelength of the waves after they pass through the gap. [1] (c) State one change the student could make to the gap to increase the spreading. [1]

(a) Diffraction. ✓

(b) The wavelength stays the same. Diffraction does not change wavelength, speed or frequency, only the shape of the wavefronts. ✓

Part (b) is the trap. Diffraction changes the shape of the wavefronts but NOT the wavelength. Many students wrongly say the wavelength decreases.

Analysis

[3 marks]

A wave crosses from one medium into another in which it travels more slowly. It meets the boundary along the normal (perpendicular to the boundary).

The wave slows down, yet it does not change direction. Explain how it is possible for the speed to change without the direction changing.

Refraction bends a wave only when one part of a wavefront enters the slower medium before another part, twisting the wavefront.
Here the wave meets the boundary head-on, so the whole wavefront crosses at the same instant.
Every part slows together, so there is no twisting and the direction stays the same. The speed (and wavelength) still change.

Direction change needs an angle. No angle, no bend, even though the speed change still happens. Speed change and direction change are separate effects.

[3 marks]

Copy and complete the table by writing "changes" or "stays the same" for the wavelength of a water wave during each behaviour.

(a) Reflection from a barrier [1] (b) Refraction into shallower water [1] (c) Diffraction through a gap [1]

(a) Reflection → wavelength stays the same ✓

(b) Refraction → wavelength changes (decreases) ✓

Only refraction changes the wavelength, because only refraction involves a change of speed. Reflection and diffraction keep the wave in the same medium, so the speed and wavelength are unchanged.