Which of the following explains why the sky is blue?

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BlackTom AI · Accepted Answer

Question restatement: The query asks which explanation accounts for why the sky appears blue. 
Option A: Planck's law. This law describes the spectral density of electromagnetic radiation emitted by a black body at a given temperature. It explains energy distribution across wavelengths, not the color of the sky, so it does not directly justify the blue sky phenomenon. 
Option B: Stefan-Boltzmann law. This law relates the total energy emitted by a black body to its temperature via the fourth power, focusing on overall power rather than scattering processes in the atmosphere. It does not address the color observed in daylight skies. 
Option C: Wien's displacement law. This law connects the peak emission wavelength to temperature for a black body, indicating that hotter objects emit shorter wavelengths. While it explains color changes with temperature, it does not explain why the sky is blue due to scattering of sunlight. 
Option D: Rayleigh scattering. This phenomenon occurs when light travels through a medium with particles much smaller than the wavelength of light. Shorter wavelengths (blue and violet) scatter more than longer wavelengths (red). Because the scattered blue light is what reaches our eyes from every direction, the sky appears blue. This explanation directly ties atmospheric scattering to the observed color. 
In terms of the underlying physics, the correct mechanism relies on the wavelength-dependent scattering cross-section proportional to 1/λ^4, which heavily favors blue wavelengths being redirected toward observers, while violet is less noticeable due to solar spectrum and atmospheric absorption. 
Why the incorrect options fail conceptually: Planck's law, Stefan-Boltzmann law, and Wien's displacement law each address black-body radiation properties and spectral characteristics of emitted light, not the interaction of sunlight with atmospheric particles and the resulting directional scattering that makes the sky blue. The question centers on scattering processes in Earth's atmosphere, not on emission spectra of sources. 
Therefore, the statement that best explains the blue sky is Rayleigh scattering, as it captures the wavelength-dependent scattering responsible for the observed color.

GEOM30009_2025_SM1 6.3 - Assessment Task 3: In-class online quiz

Which of the following explains why the sky is blue?

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