From the point of view of atmospheric optics, why does the Sun appear reddish in colour at sunrise and sunset when viewed from the Earth's surface?

Introduction / Context:
This question belongs to the topic of light scattering in the Earth's atmosphere, which is studied in physics. It explains everyday observations such as the blue colour of the sky and the reddish appearance of the Sun at sunrise and sunset. Competitive exams frequently test whether learners can correctly apply the concept of Rayleigh scattering and the dependence of scattering on wavelength to such phenomena.

Given Data / Assumptions:
The question asks why the Sun looks red during sunrise and sunset and provides four options that offer different explanations:
- The Sun emits only red light at those times
- Red light, having longer wavelength, scatters away
- The Sun comes out of the mountains
- All other colours are scattered away except red
It is assumed that the learner knows that white sunlight is composed of a spectrum of colours and that the Earth's atmosphere scatters shorter wavelengths more strongly.

Concept / Approach:
According to Rayleigh scattering, the amount of scattering by small particles in the atmosphere is inversely proportional to the fourth power of the wavelength. This means shorter wavelengths such as blue and violet are scattered more, while longer wavelengths such as red are scattered less. At sunrise and sunset, the Sun's light travels through a much longer path in the atmosphere. Most of the shorter wavelength components are scattered out of the direct line of sight, leaving the longer wavelength red and orange light to dominate the direct sunlight that reaches the observer. Therefore, the correct explanation is that all other colours are scattered away except red, making the Sun appear reddish.

Step-by-Step Solution:
Step 1: Recall that sunlight is a mixture of many colours ranging from violet to red. Step 2: Understand that when the Sun is near the horizon, its light travels through a longer column of air, encountering more dust and gas molecules. Step 3: Apply Rayleigh scattering, which states that shorter wavelengths like blue and violet are scattered much more than longer wavelengths such as red and orange. Step 4: Recognise that because the shorter wavelengths are scattered out of the line of sight, the direct rays that reach the eye are enriched in red and orange light. Step 5: Conclude that the Sun appears red at sunrise and sunset because all other colours are scattered away except the red dominated components.

Verification / Alternative check:
An alternative way to verify is to compare this reasoning with why the sky is blue at noon. At midday, the Sun is overhead and light passes through a shorter atmospheric path, so blue light is scattered in all directions, giving the sky its blue appearance. At sunrise or sunset, the Sun is low on the horizon; the scattered blue light is largely removed from the direct path, and the remaining direct sunlight is reddish. No physical mechanism exists for the Sun to emit only red light at certain times, and the presence or absence of mountains is clearly not relevant. This confirms that the explanation involving selective scattering of all colours except red is the correct one.

Why Other Options Are Wrong:
Because the Sun emits only red light at those times: The Sun consistently emits a broad spectrum of light; its emission does not suddenly change to only red at sunrise or sunset.
Because red light, having longer wavelength, scatters away: This reverses the correct reasoning; red light actually scatters less, not more.
Because the Sun comes out of the mountains: The presence of mountains may obscure the Sun, but it does not change the fundamental optical explanation for the reddish colour.

Common Pitfalls:
Learners sometimes confuse the role of scattering and mistakenly think that the colour depends on where the Sun is physically located relative to mountains or buildings. Others misunderstand Rayleigh scattering and assume that longer wavelengths must scatter more. To avoid these errors, remember that shorter wavelengths scatter much more strongly, leaving the longer wavelengths like red to dominate the direct sunlight during sunrise and sunset.

Final Answer:
The Sun appears red at sunrise and sunset because all other colours are scattered away except red, which reaches the observer more directly.