Question at position 51 The sky is blue becausemolecules scatter red light more effectively than blue light.the Sun mainly emits blue light.molecules scatter blue light more effectively than red light.the atmosphere absorbs mostly blue light.the atmosphere transmits mostly blue light.Clear my selection

Question

BlackTom AI · Accepted Answer

Molecules scatter blue light more effectively than red light, which is the physical basis for the sky appearing blue.

1. Relevant knowledge points and concepts
- Rayleigh scattering: when particles (air molecules) are much smaller than the wavelength of light, scattered intensity varies as 1/λ^4 (λ = wavelength).
- Visible sunlight is a mix of wavelengths (white light); different wavelengths are scattered differently.
- Human perception: scattered light from all directions is what we see as the sky color.

2. Step-by-step reasoning
- Sunlight entering the atmosphere contains all visible wavelengths.
- Air molecules are small relative to visible wavelengths, so Rayleigh scattering applies.
- Because scattering intensity ∝ 1/λ^4, shorter wavelengths (blue ~450 nm) are scattered far more than longer wavelengths (red ~650 nm).
- The scattered component reaching an observer from directions away from the Sun is therefore enriched in blue light, making the sky look blue.

3. Calculation with intermediate steps
- Let λ_blue = 450 nm and λ_red = 650 nm.
- Ratio of scattering (blue relative to red) = (λ_red / λ_blue)^4.
- Compute λ_red / λ_blue = 650 / 450 = 1.4444...
- Square: (1.4444)^2 ≈ 2.0864.
- Fourth power: (1.4444)^4 = (2.0864)^2 ≈ 4.352.
- Conclusion: blue light is scattered ≈4.35 times more than red light under these representative wavelengths.

4. Why the correct answer is correct
- The strong wavelength dependence (1/λ^4) quantitatively explains why shorter (blue) wavelengths dominate the scattered light and thus the perceived sky color.

5. Why the other options are incorrect
- "Molecules scatter red light more effectively" contradicts Rayleigh scattering’s 1/λ^4 dependence.
- "The Sun mainly emits blue light" is false: the Sun’s spectrum is broad (white light) peaking near green-yellow, not predominantly blue.
- "The atmosphere absorbs mostly blue light" is incorrect: the atmosphere does not preferentially absorb blue in the visible; it scatters it.
- "The atmosphere transmits mostly blue light" is misleading: transmission is not preferential for blue; blue is removed from direct beam by scattering and sent into other directions.

6. Conclusion
The correct answer is "molecules scatter blue light more effectively than red light." This matches the provided answer.

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