The Sun is observed to be reddish when it is near the horizon, i.e., in the morning and the evening. This is because

1. Visible Light and the Electromagnetic Spectrum (basic)

To understand optics, we must first understand the nature of light itself. Light is a form of Electromagnetic (EM) Radiation—energy that travels through space as waves. While we often think of light only as what we can see, it is actually part of a much broader Electromagnetic Spectrum. This spectrum includes waves ranging from massive radio waves, which can be larger than our planet, to high-frequency waves like microwaves and X-rays Physical Geography, PMF IAS, Earths Atmosphere, p.279. Light does not require a medium to travel; it can move through a vacuum, enabling us to see stars and receive energy from the Sun.

The Visible Spectrum is the tiny slice of the EM spectrum that the human eye is sensitive to. When white light (like sunlight) passes through a medium like a glass prism, it slows down and bends, splitting into a beautiful band of colors. This phenomenon reveals that white light is actually a mixture of different wavelengths Science, Class X (NCERT 2025 ed.), Chapter 10: The Human Eye and the Colourful World, p.167. Each color corresponds to a specific wavelength, which is the distance between two consecutive peaks of the wave.

The sequence of these colors is easily remembered by the acronym VIBGYOR. It is crucial to understand that these colors are arranged in order of their physical properties:

Color	Wavelength	Frequency/Energy
Red	Longest	Lowest
Violet	Shortest	Highest

Objects become visible to us because they reflect light into our eyes or transmit it if they are transparent Science, Class X (NCERT 2025 ed.), Chapter 9: Light – Reflection and Refraction, p.134. The way these different wavelengths interact with particles in our atmosphere—like dust or gas molecules—determines why we see a blue sky or a red sunset. For instance, shorter wavelengths (like blue) scatter more easily, while longer wavelengths (like red) can travel further through the atmosphere without being redirected Science, Class X (NCERT 2025 ed.), Chapter 10: The Human Eye and the Colourful World, p.169.

Sources: Physical Geography by PMF IAS, Earths Atmosphere, p.279; Science, Class X (NCERT 2025 ed.), Chapter 10: The Human Eye and the Colourful World, p.167; Science, Class X (NCERT 2025 ed.), Chapter 9: Light – Reflection and Refraction, p.134; Science, Class X (NCERT 2025 ed.), Chapter 10: The Human Eye and the Colourful World, p.169

2. Refraction and Dispersion of Light (intermediate)

When we talk about light passing through a medium, we usually think of a simple glass slab. However, a triangular glass prism behaves quite differently. Unlike a rectangular slab where the entering and exiting surfaces are parallel (causing the light to emerge parallel to its original path), a prism has lateral surfaces inclined at an angle. This angle is known as the angle of the prism Science, Class X (NCERT 2025 ed.), Chapter 10, p.165. Because the surfaces are not parallel, the light ray is bent significantly from its original path, a phenomenon that leads to one of nature's most beautiful displays: dispersion.

Dispersion is the process of splitting white light into its constituent seven colors (VIBGYOR). Why does this happen? It’s all about speed. While all colors of light travel at the same speed in a vacuum, they travel at different speeds through a medium like glass. Because their speeds differ, their refractive indices also differ, causing them to bend by different angles. Red light, having a longer wavelength, travels the fastest in glass and is refracted (bent) the least. Conversely, violet light travels the slowest and is refracted the most Science, Class X (NCERT 2025 ed.), Chapter 10, p.167.

Feature	Red Light	Violet Light
Wavelength	Longer	Shorter
Speed in Glass	Higher	Lower
Angle of Deviation	Minimum (Bends least)	Maximum (Bends most)

Sir Isaac Newton was the first to demonstrate that white light is actually a mixture of these colors. He performed a famous experiment where he placed a second, inverted prism behind the first one. The first prism dispersed the light into a spectrum, and the second inverted prism recombined those colors back into a single beam of white light Science, Class X (NCERT 2025 ed.), Chapter 10, p.167. This proved that the prism does not "create" the colors; it simply separates the components already present in sunlight.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 10: The Human Eye and the Colourful World, p.165; Science, Class X (NCERT 2025 ed.), Chapter 10: The Human Eye and the Colourful World, p.167

3. Atmospheric Refraction: Advanced Sunrise and Delayed Sunset (intermediate)

To understand why we see the Sun before it technically rises, we must first look at the Earth's atmosphere as a giant, layered lens. The air surrounding our planet is not uniform; it is densest near the surface and becomes progressively thinner (less dense) as we move toward space. When sunlight enters our atmosphere from the vacuum of space, it travels from a rarer medium to a denser medium. According to the laws of refraction, light in this scenario bends towards the normal. Because the density of air increases continuously as the light descends, the sunlight follows a curved path, bending downwards toward the Earth's surface Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.255.

This bending of light is the reason for Advanced Sunrise. By "actual sunrise," we mean the moment the Sun physically crosses the horizon. However, because the atmosphere bends the rays coming from the Sun (which is still below the horizon), those rays reach our eyes earlier than they otherwise would. Our brain, which perceives light as traveling in straight lines, traces these refracted rays back to an apparent position above the horizon. This phenomenon allows us to see the Sun approximately 2 minutes before it actually rises Science, class X (NCERT 2025 ed.), The Human Eye and the Colourful World, p.168.

The same logic applies during the evening, leading to a Delayed Sunset. Even after the Sun has physically dipped below the horizon, its light continues to bend over the curve of the Earth, keeping the Sun visible for about 2 more minutes. Consequently, the total duration of daylight is increased by roughly 4 minutes every day. Interestingly, this atmospheric refraction also causes the apparent flattening of the Sun's disc at dawn and dusk. Because the bottom edge of the Sun is lower in the atmosphere than the top edge, its rays are refracted more strongly, "lifting" the bottom more than the top and creating an oval shape Science, class X (NCERT 2025 ed.), The Human Eye and the Colourful World, p.168.

Phenomenon	Cause	Time Impact
Advanced Sunrise	Refraction through increasing air density	Seen ~2 mins early
Delayed Sunset	Refraction through increasing air density	Seen ~2 mins late
Apparent Flattening	Differential refraction of Sun's top/bottom edges	Visual distortion

Sources: Science, class X (NCERT 2025 ed.), The Human Eye and the Colourful World, p.168; Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.255

4. Total Internal Reflection and Mirages (exam-level)

Total Internal Reflection (TIR) is a fascinating optical phenomenon that occurs when light attempts to travel from a denser medium (like water or glass) to a rarer medium (like air). Under normal circumstances, light would refract away from the normal. However, as the angle of incidence increases, the angle of refraction also increases until it reaches 90°. The specific angle of incidence that results in a 90° refraction is called the Critical Angle. If the incident light hits the boundary at any angle greater than this critical angle, it doesn't pass through at all; instead, it reflects entirely back into the denser medium. Unlike ordinary reflection from a mirror, where some light is always absorbed, TIR is 100% efficient, which is why it is used in high-tech fiber optics. One of the most common natural applications of TIR is the Mirage, often seen in dry hot deserts like the Sahara or the Thar Physical Geography by PMF IAS, Climatic Regions, p.441. On a hot day, the ground becomes intensely heated, warming the layer of air directly above it. This creates a temperature gradient: the air near the ground is hot (less dense/rarer), while the air higher up is cooler (more dense/denser). As light from a distant object (like a tree or the sky) travels downward, it passes through increasingly rarer layers of air and bends further away from the normal. Eventually, the light hits a layer at an angle exceeding the critical angle and undergoes Total Internal Reflection, curving back upward toward the observer's eye. The brain, assuming light travels in straight lines, perceives this light as coming from the ground, creating a shimmering image that looks like a pool of water reflecting the sky. While TIR involves reflection, it is important to remember that the fundamental laws of reflection still hold true at the point of incidence: the angle of incidence equals the angle of reflection Science, Class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.135. Understanding the distinction between refraction (bending) and this specific type of reflection is key to mastering geometrical optics.

Condition	Status for TIR
Direction of Light	Must move from Denser to Rarer medium
Angle of Incidence	Must be greater than the Critical Angle
Energy Loss	Negligible (Total reflection)

Sources: Science, Class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.135; Physical Geography by PMF IAS, Climatic Regions, p.441

5. The Tyndall Effect and Colloids (basic)

To understand why the sky is blue or why sunset is red, we must first understand how light interacts with the tiny particles around us. This phenomenon is known as the Tyndall Effect. Imagine a dark room with a single beam of sunlight entering through a small hole; you can see the path of light clearly because it strikes colloidal particles like dust and smoke, which scatter the light toward your eyes. In a pure solution (like salt dissolved in water), the particles are too small to disturb the light, so the path remains invisible. However, in a heterogeneous mixture like our atmosphere, the presence of smoke, tiny water droplets, and air molecules makes the beam visible through diffuse reflection Science, Class X (NCERT 2025 ed.), Chapter 10, p.169.

The color of this scattered light is not random—it depends heavily on the size of the scattering particles. This is a crucial concept for your exams. Think of it as a filter system based on size:

• Very Fine Particles: These are so small that they primarily scatter shorter wavelengths, which corresponds to blue light.
• Larger Particles: These are big enough to scatter longer wavelengths, such as orange and red.
• Very Large Particles: If the particles (like thick water droplets in a cloud) are large enough, they scatter all colors of light equally, making the light appear white Science, Class X (NCERT 2025 ed.), Chapter 10, p.169.

In the natural world, you can observe this when sunlight passes through a dense forest canopy. Here, tiny water droplets in the mist act as the scattering agents, illuminating the sunbeams in a beautiful display of the Tyndall effect. From a geographical perspective, the transparency of our atmosphere is constantly affected by these aerosols (dust, soot, pollen) and water vapor. When the wavelength of light is larger than the particle, scattering occurs; however, if the particle is much larger than the wavelength, simple reflection takes over Physical Geography by PMF IAS, Horizontal Distribution of Temperature, p.283.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 10: The Human Eye and the Colourful World, p.169; Physical Geography by PMF IAS, Horizontal Distribution of Temperature, p.283

6. Rayleigh Scattering Law (exam-level)

At its core, Rayleigh Scattering is the phenomenon where light is deflected in various directions by particles that are significantly smaller than the wavelength of the light itself. Lord Rayleigh discovered that the intensity of scattered light (I) is not uniform across all colors; rather, it is inversely proportional to the fourth power of the wavelength (λ). Mathematically, this is expressed as I ∝ 1/λ⁴. This means that even a small decrease in wavelength leads to a massive increase in scattering. Since blue light has a much shorter wavelength than red light—roughly 1.8 times shorter—it is scattered about 10 times more efficiently by the fine molecules of nitrogen and oxygen in our atmosphere Science, Class X (NCERT 2025 ed.), Chapter 10, p.169.

This law explains why the clear sky appears blue. As sunlight enters the atmosphere, the shorter blue wavelengths are scattered in every direction by air molecules, eventually reaching our eyes from all parts of the sky. If Earth had no atmosphere to facilitate this scattering, the sky would appear pitch black, just as it does to astronauts in space Exploring Society: India and Beyond, Class VI (NCERT 2025 ed.), Oceans and Continents, p.28. It is important to note that Rayleigh scattering specifically requires the scattering particles (like gas molecules) to be smaller than the wavelength of light. If the particles are large (like water droplets in a cloud), they scatter all wavelengths almost equally, which is why clouds appear white.

The reddening of the Sun at sunrise and sunset is the other side of this same coin. During these times, sunlight must travel through a much thicker layer of the atmosphere to reach the observer. By the time the light reaches your eyes, most of the shorter wavelengths (blue and violet) have been scattered away and lost from the direct beam. What remains are the longer wavelengths—the reds and oranges—which are scattered the least and can pass through the dense atmospheric path relatively undisturbed Science, Class X (NCERT 2025 ed.), Chapter 10, p.169.

Sources: Science, Class X (NCERT 2025 ed.), The Human Eye and the Colourful World, p.169; Exploring Society: India and Beyond, Class VI (NCERT 2025 ed.), Oceans and Continents, p.28

7. Light Scattering at the Horizon (exam-level)

When we look at the Sun during sunrise or sunset, we aren't just seeing light; we are seeing the result of a massive atmospheric filter. To understand why the Sun appears reddish at the horizon, we must first look at Rayleigh Scattering. This principle states that the intensity of light scattering is inversely proportional to the fourth power of the wavelength (Intensity ∝ 1/λ⁴). In simpler terms, shorter wavelengths like blue and violet are scattered much more aggressively by the fine molecules in our atmosphere than longer wavelengths like red and orange Science, Class X (NCERT 2025 ed.), Chapter 10, p.169.

The magic happens because of the geometry of the Earth's atmosphere. When the Sun is directly overhead at noon, the light travels a relatively short, vertical distance through the atmosphere. However, at the horizon (sunrise or sunset), the Sun's rays must travel through a significantly thicker layer of air and a higher concentration of dust and water vapor to reach your eyes Certificate Physical and Human Geography, GC Leong, Chapter 13, p.132. During this long, oblique journey, most of the blue and shorter-wavelength light is scattered away in all directions, leaving only the least-scattered, longer-wavelength light—the reds and oranges—to reach the observer.

Position of Sun	Path Length through Atmosphere	Dominant Visual Effect
Overhead (Noon)	Shortest path	Minimal scattering; Sun appears white.
At Horizon	Longest (Slant) path	Blue light scattered away; Sun appears reddish.

It is also important to note that the horizon doesn't just change the color; it changes the perceived position. Due to Atmospheric Refraction, the Sun appears to rise about 2 minutes before it actually crosses the horizon and stays visible for about 2 minutes after it has set. This is because the atmosphere acts like a lens, bending the light toward the Earth Science, Class X (NCERT 2025 ed.), Chapter 10, p.168. This same refraction also causes the Sun's disc to appear slightly flattened at the horizon.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 10: The Human Eye and the Colourful World, p.168-169; Certificate Physical and Human Geography, GC Leong, Chapter 13: Climate, p.132

8. Solving the Original PYQ (exam-level)

This question is a classic application of Rayleigh scattering and the properties of the visible spectrum you just studied. You’ve learned that the intensity of scattering depends heavily on wavelength: shorter wavelengths scatter easily, while longer ones remain relatively undisturbed. When the Sun is near the horizon, its rays must traverse a much thicker layer of the atmosphere compared to when it is overhead at noon. By applying the building blocks of wavelength-dependency, you can see that the atmosphere acts as a filter, scattering away the "noisy" blue light and leaving only the "sturdy" red light to reach your eyes.

To arrive at the correct answer, (A) red light is least scattered by atmosphere, you must focus on the relationship between distance and wavelength. As sunlight travels that extended path through the horizon, the shorter-wavelength blue and violet lights are scattered in all directions by air molecules and fine particles. Because red light has the longest wavelength in the visible spectrum, it is least scattered and survives the long journey to your retina. As explained in Science, class X (NCERT 2025 ed.), this principle is also why red is used for danger signals—it can penetrate through distance and atmospheric interference more effectively than any other color.

UPSC often uses distractor options to test your conceptual clarity. Option (B) is a direct reversal of the scientific law, meant to catch students who confuse the relationship between scattering and wavelength. Option (C) is a tautological trap—it simply restates the observation as its own cause without providing a scientific mechanism. Option (D) incorrectly suggests emission; however, the atmosphere does not produce its own visible light in this context but merely scatters the light coming from the Sun. Distinguishing between scattering (changing the direction of light) and emission (the creation of light) is vital for navigating these types of conceptual questions.