Properties of waves questions

Properties of waves syllabus

Questions on properties of waves can test recall, calculations, explanations, diagrams, data handling and practical skills. You should be able to:

• 3.2 explain the difference between longitudinal and transverse waves
• 3.3 know the definitions of amplitude, wavefront, frequency, wavelength and period of a wave
• 3.4 know that waves transfer energy and information without transferring matter
• 3.5 know and use the relationship between the speed, frequency and wavelength of a wave: wave speed = frequency × wavelength v = f × λ
• 3.6 use the relationship between frequency and time period: frequency = 1 / time period f = 1 / T
• 3.7 use the above relationships in different contexts, including sound waves and electromagnetic waves
• 3.8 explain why there is a change in the observed frequency and wavelength of a wave when its source is moving relative to an observer and that this is known as the Doppler effect
• 3.9 explain that all waves can be reflected and refracted

How to answer properties of waves questions

1. Label wavelength, amplitude, frequency and normal lines before interpreting a diagram.
2. Use a ruler for ray diagrams and measure angles from the normal, not the surface.
3. Keep wave properties separate from applications and link each hazard to a suitable precaution.
4. For calculations, convert frequencies and distances before using wave speed relationships.

Properties of waves example questions and worked answers

These examples show how information in a diagram, graph or experimental context becomes part of a complete exam answer.

Example 1: Diffraction through different gaps

January 2018 · Paper 2P · Question 3(a)

Question 1

A student investigates different properties of waves.

He uses this ripple tank to investigate diffraction.

The ripple tank produces plane waves.

These waves hit a barrier with a gap in it.

The student keeps the wavelength constant but varies the size of the gap in the barrier.

Complete the diagrams to show what happens to the plane waves as they go through the different sized gaps in the barrier.

Final answer

Complete the diagrams as shown: strongly curved wavefronts after the small gap, and mostly straight wavefronts with curved ends after the large gap.

Mark scheme points

1. M1 Small gap: draw at least three correctly curved wavefronts centred on the gap. Their exact start and finish positions are not assessed.
2. M2 Small gap: keep the wavefront spacing consistent with the original spacing, judged by eye.
3. M3 Large gap: draw mostly straight wavefronts with curved edges. Each must be shorter than the original wavefronts. Spacing in this diagram is not assessed.

Explanation

Diffraction is the spreading of waves as they pass through a gap. Spreading is greatest when the gap is about the same size as the wavelength.

• For the small gap, draw nested semicircular wavefronts sharing a centre at the gap.
• The wavelength is unchanged, so the distance between neighbouring wavefronts stays the same.
• For the large gap, show less spreading: straight central sections with the ends bending back towards the barrier.

Common mistakes

• Increasing the spacing after the small gap. The wavefronts spread out, but their separation must still represent the original wavelength.
• Drawing full-length straight lines after the large gap. Make them shorter than the incident wavefronts and curve their ends.

Example 2: Testing a microwave intensity relationship

January 2023 · Paper 1P · Question 9(c)(ii)

Question 2

This question is about waves.

A student uses a microwave source and a receiver to investigate microwaves.

Photograph 1 shows how the student sets up their apparatus.

The meter shows the strength of the microwaves detected by the receiver.

The strength of the microwaves is measured in arbitrary units.

The student varies the distance between the microwave source and the receiver, and records the meter readings.

The graph shows the results of the student’s investigation.

The student concludes that the meter reading is inversely proportional to the distance between the microwave source and the receiver.

To be inversely proportional

meter reading × distance = constant

Comment on the student’s conclusion.

You should use data from the graph in your answer.

Final answer

The student’s conclusion is incorrect: the meter reading is not inversely proportional to the distance. From the graph, at 10 cm the reading is approximately 100, giving a product of 100 × 10 = 1000. At 20 cm the reading is approximately 25, giving a product of 25 × 20 = 500. These products are different, so meter reading × distance is not constant.

Mark scheme points

1. M1 State that the relationship is not inversely proportional, or that the student’s conclusion is incorrect.
2. M2 Calculate the product for one pair of graph readings: 100 × 10 = 1000.
3. M3 Calculate the product for a second pair of graph readings: approximately 25 × 20 = 500.
4. M4 State that meter reading × distance is not constant.

Explanation

To test inverse proportionality, multiply the meter reading by its corresponding distance at two points on the graph.

At 10 cm: reading × distance ≈ 100 × 10 = 1000
At 20 cm: reading × distance ≈ 25 × 20 = 500

For inverse proportionality, both products must be the same. Here, the products differ substantially, so the data do not support the student’s conclusion.

Common mistakes

• The examiner reported that nearly half of candidates did not use graph data despite being asked to. Quote two distance–reading pairs and show both products.
• A decreasing curved graph does not by itself prove inverse proportionality. Check whether the products are constant before making your conclusion.

Practise Properties of waves questions

Build a focused practice set from this part of the Edexcel IGCSE Physics syllabus.