Light & the Electromagnetic Spectrum — Practice · Grade 8 Science

Chapter: Waves, Sound & Light

What this practice covers

By the end of Light & the Electromagnetic Spectrum you should be able to:

11 questions, numeric, multiple choice and written answer, each with a worked explanation.

Vocabulary used: electromagnetic (EM) wave, electromagnetic spectrum, reflection, absorption, transmission, model, evidence, system boundary, assumption.

Problem 1

A radio wave has a frequency of 1,000,000 Hz. Using the speed of light as 300,000,000 m/s, find its wavelength.
Answer and explanation

300 m

λ = v ÷ f = 300,000,000 ÷ 1,000,000 = 300 m.

Problem 2

Which type of EM wave has the LONGEST wavelength?
  1. gamma rays
  2. visible light
  3. radio waves
  4. X-rays
Answer and explanation

radio waves

Radio waves sit at the long-wavelength end of the electromagnetic spectrum.

Problem 3

Which listed electromagnetic region has the highest energy PER PHOTON?
  1. radio waves
  2. microwaves
  3. visible light
  4. gamma rays
Answer and explanation

gamma rays

Gamma rays have higher photon energy because their frequencies are higher. Total delivered energy also depends on how much radiation arrives and for how long.

Problem 4

When light passes through clear glass, most of it is…
  1. reflected
  2. absorbed
  3. transmitted
  4. turned into sound
Answer and explanation

transmitted

Clear glass mostly transmits light, letting it pass through.

Problem 5

Explain why we see a red apple as red.
Answer and explanation

The apple's surface absorbs most colors of light but reflects red wavelengths back to our eyes, so we perceive the apple as red.

Problem 6

A pencil looks bent when placed in a glass of water because light…
  1. is absorbed at the surface
  2. is only reflected
  3. refracts (bends) when passing from water to air
  4. stops moving inside the water
Answer and explanation

refracts (bends) when passing from water to air

Light bends (refracts) as it passes between materials like water and air, making the pencil appear bent.

Problem 7

A red shirt appears bright under white light but dark under blue light. Explain without changing the shirt's material. Explain the reasoning in your own words.
Answer and explanation

The reflected light depends on both the material and the illumination.

White light contains red wavelengths that the material can reflect toward the eye.

Blue illumination provides little red light to reflect and much may be absorbed.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 8

White light splits into colors through a prism. What property differs among the colors? Show the reasoning, not just the conclusion.
Hints

Compare speed in the same medium; do not confuse frequency with speed.

Answer and explanation

Frequency and wavelength differ; frequency remains unchanged when light passes between stationary media, while speed and wavelength can change.

Problem 9

Why can sunlight cross space while ordinary sound cannot?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Light is electromagnetic and needs no material medium; sound is a mechanical disturbance requiring matter.

Problem 10

Energy per photon differs from total energy arrivingEnergy per photon differs from total energy arrivingLower-frequency caseEnergy per photon: 1 relative unit(s)Total for matched area/time: 8Higher-frequency caseEnergy per photon: 2 relative unit(s)Total for matched area/time: 6
The deliberately simplified lower-frequency case has eight photons at one relative energy unit each; the higher-frequency case has three at two units each. Totals are eight and six under the matched area/time comparison. The circles are a counting aid rather than literal photon shapes. Higher frequency means higher energy per photon, but total exposure also depends on how many arrive and the comparison conditions.
For this unit’s illustrated case, which case has more energy per photon, and which has more total supplied energy for the matched comparison?
Answer and explanation

The higher-frequency case has more per photon (2 versus 1). The lower-frequency case has more total (8 versus 6), because it has more supplied photons.

A photon is a quantum of electromagnetic radiation. Per-photon energy, arrival amount and exposure duration are distinct. Do not replace a total-energy comparison with a frequency-only slogan when the photon counts or duration also change.

The higher-frequency case has more per photon (2 versus 1). The lower-frequency case has more total (8 versus 6), because it has more supplied photons.

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 11

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.

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