Waves, Sound & Light — Chapter quiz · Physical Science: Matter, Forces & Energy, Grade 8

Connect vibrating sources, wave properties, sound, light and information with a complete source–medium–receiver model. This is a required chapter and is included in the course final.

Problem 1

Illustrative sources A/B/C have amplitudes 1/3/1 mm and complete 4/4/6 cycles respectively in 0.02 s. Compare frequency and amplitude and explain what a receiver prediction would need.
Answer and explanation

A and B are 200 Hz, while C is 300 Hz. B has three times A’s amplitude; C has A’s amplitude but higher frequency. Frequency and amplitude are distinct. Predicting received sound also needs distance, medium, losses and receiver response. The source records alone do not establish an exact perceived loudness.

A and B are 200 Hz, while C is 300 Hz. B has three times A’s amplitude; C has A’s amplitude but higher frequency. Frequency and amplitude are distinct. Predicting received sound also needs distance, medium, losses and receiver response. The source records alone do not establish an exact perceived loudness.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 2

What is missing from a drawing with only a source and an ear?
Answer and explanation

The intermediate medium and interactions, and how the receiver responds. The picture should also identify limits such as scale and omitted biological detail.

The intermediate medium and interactions, and how the receiver responds. The picture should also identify limits such as scale and omitted biological detail.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 3

Independent chapter check: A source-position trace has peaks 0.2 s apart and swings from −3 to +3 mm around zero. Find period, frequency and amplitude, and distinguish amplitude from peak-to-trough span.
Answer and explanation

Period 0.2 s; frequency 5 Hz; amplitude 3 mm; peak-to-trough span 6 mm. The source moves locally rather than traveling across the sound path.

Use matching-peak separation for period and its reciprocal for frequency.

Measure amplitude from the middle position to a peak.

Period 0.2 s; frequency 5 Hz; amplitude 3 mm; peak-to-trough span 6 mm. The source moves locally rather than traveling across the sound path.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 4

Illustrative waves in the same medium travel at 12 m/s. A has frequency 3 Hz and B 6 Hz. Their amplitudes are equal. Find wavelengths, distinguish spatial from time graphs and identify a limit of an energy comparison based on amplitude alone.
Answer and explanation

Using wavelength=speed/frequency, A is 4 m and B 2 m. A displacement–distance graph shows wavelength; a displacement–time graph shows period and frequency at a location. Equal amplitude does not generally establish equal energy for different frequencies or wave conditions; specify the medium, frequency and model for energy comparisons.

Using wavelength=speed/frequency, A is 4 m and B 2 m. A displacement–distance graph shows wavelength; a displacement–time graph shows period and frequency at a location. Equal amplitude does not generally establish equal energy for different frequencies or wave conditions; specify the medium, frequency and model for energy comparisons.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 5

Can two waves have equal frequency but different amplitude?
Answer and explanation

Yes. Cycle rate and maximum displacement describe different features. Their energy transfers may differ under otherwise comparable conditions.

Yes. Cycle rate and maximum displacement describe different features. Their energy transfers may differ under otherwise comparable conditions.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 6

Independent chapter check: A supplied wave has wavelength 0.8 m and frequency 5 Hz. Find wave speed. If its matched-model amplitude doubles at the same medium/frequency/duration, what happens to the squared-amplitude energy comparison?
Answer and explanation

Speed 0.8×5 = 4 m/s. The represented energy increases by a factor of four under the stated matched squared-amplitude comparison.

Combine a spatial repeat and a temporal repeat.

Keep all named comparison conditions fixed before using the amplitude pattern.

Speed 0.8×5 = 4 m/s. The represented energy increases by a factor of four under the stated matched squared-amplitude comparison.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 7

An illustrative echo returns 0.30 s after a short sound. The modeled sound speed is 340 m/s, and the path goes from source to wall and back. Find one-way distance and explain the medium’s role. Distinguish particle motion from the travelling disturbance.
Answer and explanation

The round-trip path is 340×0.30=102 m, so one-way distance is 51 m. Ordinary sound needs matter, with successive compressions and expansions transmitting a disturbance. Individual air particles mainly oscillate near their positions rather than travel the complete path. The arithmetic assumes the stated speed and clear timing.

The round-trip path is 340×0.30=102 m, so one-way distance is 51 m. Ordinary sound needs matter, with successive compressions and expansions transmitting a disturbance. Individual air particles mainly oscillate near their positions rather than travel the complete path. The arithmetic assumes the stated speed and clear timing.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 8

When should you divide speed×time by two?
Answer and explanation

When the measured time includes a round trip from the colocated source/receiver to a reflector and back, under the stated path and speed assumptions.

When the measured time includes a round trip from the colocated source/receiver to a reflector and back, under the stated path and speed assumptions.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 9

Independent chapter check: An echo returns after 0.3 s in a supplied air model with sound speed 340 m/s. Find the reflector distance and explain why using the full 102 m as the one-way distance is wrong.
Answer and explanation

Round-trip length is 340×0.3 = 102 m, so reflector distance is 51 m. The measured time includes travel out and back.

Draw both travel legs.

Divide total path by two only under the stated same-path out-and-back comparison.

Round-trip length is 340×0.3 = 102 m, so reflector distance is 51 m. The measured time includes travel out and back.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 10

Illustrative light energy incident on a sheet is 100 units: 25 reflected, 60 transmitted and 15 absorbed. A ray drawing shows only a transmitted arrow. Complete the diagram and energy account, then explain why light crossing a vacuum differs from ordinary sound.
Answer and explanation

The three pathways sum to 100 units; add reflected and absorbed arrows with their quantities. Absorbed energy can contribute to thermal changes. Light is an electromagnetic disturbance and can cross a vacuum, while ordinary sound requires material interactions. A ray model shows paths and energy partitions but not every wave or microscopic effect.

The three pathways sum to 100 units; add reflected and absorbed arrows with their quantities. Absorbed energy can contribute to thermal changes. Light is an electromagnetic disturbance and can cross a vacuum, while ordinary sound requires material interactions. A ray model shows paths and energy partitions but not every wave or microscopic effect.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 11

Why might a red object look dark under nearly blue-only light?
Answer and explanation

There is little red light available to reflect, and the object absorbs much of the incident blue light. Appearance depends on both source spectrum and surface properties.

There is little red light available to reflect, and the object absorbs much of the incident blue light. Appearance depends on both source spectrum and surface properties.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 12

Independent chapter check: For the same measured incoming 120 energy-units, a sheet reflects 30 and transmits 70. Find the absorbed amount in the stated complete account and explain why color under one lamp does not fix appearance under every lamp.
Answer and explanation

Absorbed amount is 120−30−70 = 20 units. Appearance depends on the incident wavelengths and the surface’s reflection toward the observer; a different lamp can change it.

Keep reflected, transmitted and absorbed pathways distinct.

Name both source and surface in a color explanation.

Absorbed amount is 120−30−70 = 20 units. Appearance depends on the incident wavelengths and the surface’s reflection toward the observer; a different lamp can change it.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 13

Illustrative received values are 0.15, 0.85, 0.48 and 0.88 with threshold 0.5. A drawing shows electrical input jumping directly to an ear with no speaker or air. Decode the values, identify the less robust reading and revise the drawing with intermediate interactions.
Answer and explanation

The decoded sequence is 0101. The 0.48 value is closest to the threshold, so small noise could change its symbol. Add the coil–magnet interaction, attached membrane vibration, successive air interactions and receiver. The drawing should not imply that charge or air particles travel the entire information path unchanged.

The decoded sequence is 0101. The 0.48 value is closest to the threshold, so small noise could change its symbol. Add the coil–magnet interaction, attached membrane vibration, successive air interactions and receiver. The drawing should not imply that charge or air particles travel the entire information path unchanged.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 14

What distinguishes the sound pathway from the magnetic pathway?
Answer and explanation

Sound requires material interactions in a medium. Magnetic interactions can occur across a vacuum; the two effects do not share the same air requirement.

Sound requires material interactions in a medium. Magnetic interactions can occur across a vacuum; the two effects do not share the same air requirement.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 15

Independent chapter check: A continuous signal is recorded every 0.1 s and rounded to only three allowed levels. Name two kinds of lost detail and explain why merely taking samples faster does not solve both.
Answer and explanation

Changes between sample times and finer level differences are lost. Faster sampling improves temporal coverage but does not add more allowed amplitude levels; the encoding and reconstruction also matter.

Separate sampling times from quantized levels.

State which omitted detail each proposed change addresses.

Changes between sample times and finer level differences are lost. Faster sampling improves temporal coverage but does not add more allowed amplitude levels; the encoding and reconstruction also matter.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

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