Waves, Sound & Light — Problem set, 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.

Review the key ideas

Sound Sources, Vibrations & Receivers

Amplitude and frequency can vary independently. Compare graph height and cycles per stated time separately. A and B are 200 Hz, while C is 300 Hz.

Wave Properties

Wave speed = frequency × wavelength (v = f × λ); amplitude relates to energy, and period is the inverse of frequency (T = 1/f).

Sound Waves

Sound is a longitudinal wave of compressions and rarefactions that needs a medium; frequency controls pitch, and amplitude controls loudness.

Light & the Electromagnetic Spectrum

The EM spectrum ranges from long, low-energy radio waves to short, high-energy gamma rays; light hitting matter is reflected, absorbed, or transmitted.

Waves, Information & a Speaker System

Noise can cause a reading to cross the decision threshold; a code and receiver design influence error detection and reliability. The decoded sequence is 0101.

Problem 1

What is B’s amplitude and what quantity remains unchanged?
Answer and explanation

Amplitude is 3 mm. The cycle count per time, and hence frequency of 200 Hz, is unchanged in this comparison.

Amplitude is 3 mm. The cycle count per time, and hence frequency of 200 Hz, is unchanged in this comparison.

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

Which source has the highest modeled pitch and which the largest amplitude?
Answer and explanation

C has the highest frequency, 300 Hz; B has the largest displacement amplitude, 3 mm. The two properties are independent in this comparison.

C has the highest frequency, 300 Hz; B has the largest displacement amplitude, 3 mm. The two properties are independent in this comparison.

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

Visual-model chapter review: 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

Why is crest-to-trough height not the amplitude?
Answer and explanation

Amplitude measures maximum displacement from equilibrium. For a symmetric wave the full crest-to-trough height is twice the amplitude.

Amplitude measures maximum displacement from equilibrium. For a symmetric wave the full crest-to-trough height is twice the amplitude.

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 you count hertz from this snapshot alone?
Answer and explanation

No. Frequency requires cycles per time, not merely the number of crests in a spatial drawing.

No. Frequency requires cycles per time, not merely the number of crests in a spatial drawing.

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

Visual-model chapter review: 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

What changes if a tone’s amplitude increases while frequency stays fixed?
Answer and explanation

Under comparable conditions its intensity and perceived loudness generally increase, while its basic pitch remains linked to the unchanged frequency.

Under comparable conditions its intensity and perceived loudness generally increase, while its basic pitch remains linked to the unchanged frequency.

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

Do these dots need to travel from speaker to ear?
Answer and explanation

No. Successive interactions can transfer the disturbance while the particles mainly move near their local positions.

No. Successive interactions can transfer the disturbance while the particles mainly move near their local positions.

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

Visual-model chapter review: 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

Why is frequency alone insufficient to judge total delivered energy?
Answer and explanation

Frequency sets energy per photon, while photon quantity, intensity, and exposure duration affect total delivery. Interaction with material also matters.

Frequency sets energy per photon, while photon quantity, intensity, and exposure duration affect total delivery. Interaction with material also matters.

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 is a transmitted arrow alone incomplete here?
Answer and explanation

It omits the reflected 25 and absorbed 15 units, preventing a complete energy account.

It omits the reflected 25 and absorbed 15 units, preventing a complete energy account.

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

Visual-model chapter review: 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

What must two people agree on before flashes form a reliable message?
Answer and explanation

The encoding rules, timing and how to distinguish message boundaries and symbols; the physical carrier alone does not define the meaning.

The encoding rules, timing and how to distinguish message boundaries and symbols; the physical carrier alone does not define the meaning.

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 fails if the ordinary sound path becomes a vacuum gap?
Answer and explanation

The air-medium sound transfer fails even though magnetic interactions can still occur across a vacuum. The model needs separate mechanisms for those links.

The air-medium sound transfer fails even though magnetic interactions can still occur across a vacuum. The model needs separate mechanisms for those links.

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

Visual-model chapter review: 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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