When the Sun is near the horizon during sunrise or sunset, it appears reddish in colour. Which phenomenon of light is mainly responsible for this observation?

Introduction / Context:
The reddish appearance of the Sun near the horizon at sunrise and sunset is a classic question in wave optics and atmospheric physics. It helps students understand how Earth atmosphere affects the light coming from the Sun. The key idea is that different colours of light are affected differently when they pass through air and its suspended particles. This question focuses on identifying the main optical phenomenon responsible for the reddish colour seen near the horizon.

Given Data / Assumptions:
• The Sun is observed when it is close to the horizon, either in the early morning or late evening. • Sunlight has to travel a longer path through the atmosphere at these times than at noon. • The atmosphere contains molecules, dust, and other small particles that interact with sunlight. • We assume normal clear sky conditions without extreme pollution or clouds blocking the Sun.

Concept / Approach:
White sunlight is a mixture of many colours, each corresponding to a different wavelength. Shorter wavelengths (blue and violet) are scattered much more strongly by air molecules than longer wavelengths (red and orange). This wavelength dependent scattering by molecules and tiny particles is called Rayleigh scattering. When the Sun is near the horizon, sunlight passes through a much thicker layer of atmosphere, so most of the blue and green components are scattered out of the direct path. The light that finally reaches our eyes directly from the Sun is therefore richer in red and orange components, making the Sun appear reddish. The dominant phenomenon here is scattering of light, not reflection, refraction, or dispersion in a prism.

Step-by-Step Solution:
Step 1: Recognise that at sunrise and sunset, sunlight travels a longer path through the atmosphere compared to midday. Step 2: Recall that atmospheric particles scatter shorter wavelength light (blue and violet) more strongly than longer wavelength light (red). Step 3: Understand that because of this scattering, much of the blue and green light is removed from the direct beam before it reaches the observer. Step 4: The remaining direct sunlight is dominated by red and orange wavelengths, causing the Sun to appear reddish. Step 5: Conclude that the main phenomenon involved is scattering of light in the atmosphere.

Verification / Alternative check:
The same scattering mechanism explains why the sky appears blue during the day. When the Sun is high, we receive scattered blue light from all directions while looking away from the Sun. Near the horizon, the light has travelled more through the atmosphere, so so much of the blue light has been scattered away that the Sun itself appears red. No significant refraction or dispersion processes are required to explain the colour, although small refraction effects do shift the apparent position of the Sun slightly. This reinforces that scattering is the correct answer.

Why Other Options Are Wrong:
Option a (Reflection of light): Reflection occurs at surfaces like mirrors or water, and is not the dominant effect in the open sky between the Sun and observer. Option b (Refraction of light): Refraction bends the path of light when it passes between media of different densities; it slightly changes the apparent position of the Sun but does not primarily cause the red colour. Option c (Dispersion of light): Dispersion is splitting of white light into different colours by a prism or raindrops. It explains rainbows, not the overall reddish tint of the Sun at the horizon. Option e (Total internal reflection of light): This is a phenomenon where light is completely reflected inside a denser medium at large incidence angles and is not relevant to sunlight travelling through air.

Common Pitfalls:
Many students confuse dispersion and scattering, thinking that because we see colours, dispersion must be involved. However, dispersion requires a medium like a prism where different wavelengths refract by different amounts. In the case of the reddish Sun, the main process is selective scattering by atmospheric particles. Another confusion is to attribute almost every optical effect in air to refraction, but here refraction is minor compared to scattering. Remember: blue sky and red sunsets are classic signatures of Rayleigh scattering.

Final Answer:
The reddish appearance of the Sun near the horizon is mainly due to scattering of light.