Light and sound questions

Light and sound syllabus

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

• 3.14 know that light waves are transverse waves and that they can be reflected and refracted
• 3.15 use the law of reflection (the angle of incidence equals the angle of reflection)
• 3.16 draw ray diagrams to illustrate reflection and refraction
• 3.17 practical: investigate the refraction of light, using rectangular blocks, semi-circular blocks and triangular prisms
• 3.18 know and use the relationship between refractive index, angle of incidence and angle of refraction: n = sin i / sin r
• 3.19 practical: investigate the refractive index of glass, using a glass block
• 3.20 describe the role of total internal reflection in transmitting information along optical fibres and in prisms
• 3.21 explain the meaning of critical angle c
• 3.22 know and use the relationship between critical angle and refractive index: sin c = 1 / n
• 3.23 know that sound waves are longitudinal waves that can be reflected and refracted
• 3.24P know that the frequency range for human hearing is 20–20 000 Hz
• 3.25P practical: investigate the speed of sound in air
• 3.26P understand how an oscilloscope and microphone can be used to display a sound wave
• 3.27P practical: investigate the frequency of a sound wave using an oscilloscope
• 3.28P understand how the pitch of a sound relates to the frequency of vibration of the source
• 3.29P understand how the loudness of a sound relates to the amplitude of vibration of the source

How to answer light and sound 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.

Light and sound 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: Matching reflection and refraction diagrams

November 2020 · Paper 1P · Question 1(a)

Question 1

This question is about wave behaviour.

The boxes give three types of wave behaviour and the diagrams give three examples of wave behaviour.

Draw one straight line from each wave behaviour to the example that best shows that behaviour.

Final answer

Draw the three straight connections shown:

Other acceptable complete matching patterns: either alternative below also earns full marks. You only need one complete pattern.

• Alternative 1: connect reflection to the second example instead.

• Alternative 2: connect reflection to both the first and second examples.

Mark scheme points

1. M1 Connect total internal reflection to the second example only: the ray reflects inside glass at the glass–air boundary.
2. M2 Connect refraction to the third example only: the ray changes direction as it enters glass from air.
3. M3 Connect reflection to the first example: the ray reflects at aluminium. A connection to the second example, or connections to both the first and second examples only, is also accepted.

Explanation

• Total internal reflection: check that the ray stays inside the glass after reaching the glass–air boundary. This happens when the angle of incidence is greater than the critical angle.
• Refraction: check that the ray crosses the boundary into glass and bends. Light changes speed when it passes from air into glass.
• Reflection: check that the ray bounces back from the aluminium surface. Total internal reflection is also a type of reflection, which is why the second example is accepted for this label too.

Common mistakes

• The examiner reported that a minority confused reflection and total internal reflection and earned only two marks. For total internal reflection, choose the ray reflecting inside glass, not the ray reflecting from aluminium.
• Do not draw two lines from either total internal reflection or refraction: this loses the mark for that behaviour. The allowance for two connections applies only to reflection, linked to the first and second examples.

Example 2: Changing oscilloscope settings

November 2021 · Paper 2P · Question 8(b)(ii)

Question 2

The photograph shows an oscilloscope.

An oscilloscope can be used to determine the frequency of a sound wave.

Diagram 1 shows an oscilloscope screen when a sound wave is detected.

It also shows the settings of the oscilloscope.

The oscilloscope settings are changed, as shown in Diagram 2.

On Diagram 2, draw the wave that would be displayed on the oscilloscope screen if the same sound wave is detected using these new settings.

Final answer

Draw a smooth sinusoidal wave with an amplitude of 4 squares and a period of 4 squares, as shown.

Mark scheme points

1. M1 The wave has an amplitude of 4 squares, measured from the centre line to a peak.
2. M2 The wave has a period of 4 squares, measured horizontally between adjacent peaks.

Explanation

The sound wave is unchanged, so its amplitude and period stay the same. Only the number of squares used to display them changes.

Original amplitude = 2 squares × 2 V/square = 4 V
New amplitude = 4 V ÷ (1 V/square) = 4 squares

Original period = 8 squares × 5 × 10⁻⁶ s/square = 4 × 10⁻⁵ s
New period = (4 × 10⁻⁵ s) ÷ (1 × 10⁻⁵ s/square) = 4 squares

Check your drawing:

• Peaks are 4 squares above the centre line and troughs are 4 squares below it.
• Adjacent peaks are 4 squares apart horizontally.

Common mistakes

• Most candidates drew the correct four-square period, but fewer scaled the amplitude correctly. Check both settings: halving the volts per square doubles the displayed amplitude.
• Do not confuse amplitude with peak-to-trough height. The amplitude is 4 squares; the total peak-to-trough height is 8 squares.

Practise Light and sound questions

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