Question

Why is the sky orange and red at sunrise and sunset?

Gases absorb orange and red light.
Blue wavelengths are absorbed by the atmosphere.
The thick atmosphere scatters all but longer wavelengths.
Only short wavelengths are radiated from the Sun in the morning and evening.

Answer 1

Correct answer: 3. "The thick atmosphere scatters all but longer wavelengths."

Why: At sunrise and sunset sunlight travels through a much longer path in the atmosphere. Rayleigh scattering (intensity ∝ 1/λ^4) scatters short wavelengths (blue, violet) far more strongly than long wavelengths (red, orange). The short-wavelength light is scattered out of the direct beam, so the light reaching your eyes from the Sun and the horizon is dominated by the longer red/orange wavelengths. Particles, dust and aerosols (Mie scattering) can enhance the effect and make colors even redder.

Why the others are wrong:
- Gases generally do not preferentially absorb the red/orange light in this context — scattering, not absorption, is the main cause.
- Blue light is scattered, not absorbed.
- The Sun emits the full visible spectrum at all times; the composition of the emitted light doesn’t change at sunrise/sunset.