During sunries and sunset, sun appears raddish- orange because

1. The Visible Spectrum: VIBGYOR and Wavelengths (basic)

To understand optics, we must first understand that what we perceive as 'white' light is actually a magnificent cocktail of different colors. When white light passes through a transparent medium like a glass prism, it undergoes dispersion—the process of splitting into its constituent colors Science, class X (NCERT 2025 ed.), Chapter 10: The Human Eye and the Colourful World, p.167. This happens because different colors of light travel at different speeds in glass, causing them to bend (refract) at different angles. Violet light bends the most, while red light bends the least.

The resulting band of colors is called a spectrum. We use the famous acronym VIBGYOR to remember the sequence: Violet, Indigo, Blue, Green, Yellow, Orange, and Red. The fundamental physical property that distinguishes these colors is their wavelength. Red light sits at the 'long' end of the visible spectrum, with a wavelength approximately 1.8 times greater than that of blue light Science, class X (NCERT 2025 ed.), Chapter 10: The Human Eye and the Colourful World, p.169. Conversely, Violet has the shortest wavelength and the highest frequency among the colors visible to the human eye.

Feature	Violet End	Red End
Wavelength	Shorter	Longer
Bending (in Prism)	Most (Maximum)	Least (Minimum)
Scattering	High (Scatters easily)	Low (Travels further)

This difference in wavelength has profound effects on our world. In the atmosphere, fine particles scatter shorter wavelengths (blue/violet) much more strongly than longer wavelengths (red). This is why the clear sky appears blue. Even in biology, the spectrum is critical; for instance, plants primarily absorb the red and blue parts of the spectrum for photosynthesis, while other colors are less effective for their growth Environment, Shankar IAS Academy (10th ed.), Plant Diversity of India, p.197.

Sources: Science, class X (NCERT 2025 ed.), Chapter 10: The Human Eye and the Colourful World, p.167; Science, class X (NCERT 2025 ed.), Chapter 10: The Human Eye and the Colourful World, p.169; Environment, Shankar IAS Academy (10th ed.), Plant Diversity of India, p.197

2. Atmospheric Refraction: Apparent Position and Time (intermediate)

To understand why we see the Sun before it actually rises, we must first look at the nature of our atmosphere. The Earth's atmosphere is not a uniform block of air; it is composed of layers that become progressively denser as we move from space toward the ground. When light from a celestial body (like a star or the Sun) enters this atmosphere, it travels from a vacuum into a medium with a gradually increasing refractive index. This causes the light to bend towards the normal, a phenomenon known as atmospheric refraction Science, class X (NCERT 2025 ed.), Chapter 10, p.168. Because of this continuous bending, the light reaches our eyes along a curved path. However, our brains are hardwired to perceive light as traveling in straight lines. When we trace that light back, the object appears to be at a slightly different apparent position, usually higher in the sky than its actual physical location. This effect is most pronounced near the horizon, where light has to travel through the thickest layers of the atmosphere at a slanted angle Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.255. This shift in position has a fascinating impact on our perception of time. We see the Sun about 2 minutes before it actually crosses the horizon (advanced sunrise) and continue to see it for about 2 minutes after it has physically dipped below the horizon (delayed sunset). In total, atmospheric refraction gifts us approximately 4 minutes of extra daylight every day! Additionally, you may notice the Sun looks like a flattened disc or an oval during these times; this is because the bottom edge of the Sun is closer to the horizon than the top edge, causing the light from the bottom to be refracted (lifted) more than the light from the top Science, class X (NCERT 2025 ed.), Chapter 10, p.168.

Sources: Science, class X (NCERT 2025 ed.), Chapter 10: The Human Eye and the Colourful World, p.168; Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.255

3. Dispersion of Light and Rainbow Formation (basic)

When we look at sunlight, it appears white, but it is actually a beautiful blend of seven distinct colors. The phenomenon where white light splits into its component colors is known as dispersion. This happens because light of different colors (wavelengths) travels at different speeds when it enters a transparent medium like a glass prism or a water droplet. Since the speed changes differently for each color, they each bend (refract) through different angles. As noted by Isaac Newton, who first used a prism to study this, red light bends the least while violet light bends the most, creating a distinct band of colors called a spectrum Science, Chapter 10, p.167.

A rainbow is a magnificent natural application of this principle. After a rain shower, tiny water droplets suspended in the atmosphere act like miniature prisms. The formation of a rainbow involves a specific three-step sequence of light interacting with these droplets:

• Refraction and Dispersion: As sunlight enters the droplet, it slows down and bends, splitting into its constituent colors.
• Internal Reflection: The light hits the back surface of the droplet and reflects internally.
• Refraction: The light bends once more as it exits the droplet, further separating the colors before they reach your eyes Science, Chapter 10, p.167.

For you to see this spectrum, the geometry must be precise: the Sun must be behind you, and the water droplets must be in front of you. This is why a rainbow is always formed in the direction opposite to that of the Sun. Interestingly, you don't need a storm to see this; the same physics applies when you look at the mist of a waterfall or a garden fountain on a sunny day Science, Chapter 10, p.167.

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

4. Total Internal Reflection (TIR) and Mirages (intermediate)

To understand a mirage, we must first master the phenomenon of Total Internal Reflection (TIR). Imagine light traveling from an optically denser medium (like water) to a rarer medium (like air). As the light exits, it bends away from the normal. If we gradually increase the angle of incidence, the refracted ray bends further and further until it skims the surface at a 90° angle. This specific angle of incidence is called the Critical Angle. If the light hits the boundary at an angle even slightly greater than this critical angle, it cannot escape into the rarer medium at all. Instead, it is reflected back into the denser medium as if the boundary were a perfect mirror Science, Class X, Chapter 9, p.135.

A mirage is a naturally occurring optical illusion caused by this very principle. In a hot desert, the sun heats the ground intensely, which in turn heats the layer of air in direct contact with it. This creates a temperature gradient: the air near the ground is hot (less dense/rarer), while the air layers above it are progressively cooler (more dense/denser). As light from the sky or a distant tree travels downward, it passes from denser to rarer air layers. Following the laws of refraction, the light bends progressively away from the normal Science, Class VIII, Chapter 10, p.158. Eventually, the angle of incidence becomes greater than the critical angle, and Total Internal Reflection occurs.

The light ray then curves upward toward our eyes. Because our brain perceives light as traveling in a straight line, we trace the ray back to the ground. This makes it appear as though the sky is being reflected on the surface of the earth, creating the shimmering illusion of a pool of water. This is often seen on hot tar roads or in vast desert expanses like the Sahara or the Thar Physical Geography by PMF IAS, Chapter 21, p.283.

Feature	Refraction	Total Internal Reflection (TIR)
Medium Change	Denser to Rarer or vice versa	Must be Denser to Rarer
Angle Condition	Any angle (except normal)	Angle of incidence > Critical Angle
Outcome	Light passes through and bends	Light is entirely reflected back

Sources: Science, Class X, Light – Reflection and Refraction, p.135; Science, Class VIII, Light: Mirrors and Lenses, p.158; Physical Geography by PMF IAS, Horizontal Distribution of Temperature, p.283

5. Scattering of Light and the Tyndall Effect (basic)

When we look at a clear blue sky or watch a beam of sunlight pierce through a dusty room, we are witnessing the scattering of light. In its simplest form, scattering occurs when light hits an obstacle—like a molecule of air or a speck of dust—and is redirected in various directions. This is why the path of a light beam becomes visible to us; the particles reflect the light diffusely into our eyes Science, class X (NCERT 2025 ed.), The Human Eye and the Colourful World, p.169.

The Tyndall Effect is a specific type of scattering caused by colloidal particles or very fine suspensions. You can see this when sunlight passes through a dense forest canopy, where tiny water droplets in the mist scatter the light Science, class X (NCERT 2025 ed.), The Human Eye and the Colourful World, p.169. Interestingly, the color of the light we see depends heavily on the size of the scattering particles. While larger particles (like dust or water droplets) scatter all wavelengths of light almost equally—often making the light appear white—very fine particles (like nitrogen and oxygen molecules in our atmosphere) are much better at scattering shorter wavelengths, such as blue and violet.

This relationship between wavelength and scattering is fundamental to understanding our atmosphere. According to Rayleigh scattering, the intensity of scattering is inversely proportional to the fourth power of the wavelength (Intensity ∝ 1/λ⁴). Since blue light has a shorter wavelength than red light, it is scattered much more strongly. This is why the sky appears blue. However, during sunrise and sunset, sunlight travels through a much thicker layer of the atmosphere to reach our eyes. During this long journey, most of the blue light is scattered away and lost from our line of sight, leaving behind the longer wavelengths—the reds and oranges—to reach us Physical Geography by PMF IAS, Horizontal Distribution of Temperature, p.283.

Particle Size	Predominant Scattering	Visual Result
Very Fine (Gas molecules)	Short wavelengths (Blue/Violet)	Blue Sky
Medium (Fine dust/smoke)	Depends on size; can be selective	Hazy blue or greyish tint
Large (Water droplets)	All wavelengths (Equal scattering)	White clouds/mist

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

6. Rayleigh Scattering Law (intermediate)

To understand why our world is painted in such vivid colors, we must look at Rayleigh Scattering. This phenomenon occurs when light interacts with particles (like oxygen or nitrogen molecules) that are much smaller than the wavelength of visible light. Unlike reflection, which bounces light off a surface, scattering involves the absorption and re-radiation of light in all directions.

The core of this concept is the Rayleigh Scattering Law, which states that the intensity of scattered light (I) is inversely proportional to the fourth power of its wavelength (λ). Mathematically, this is represented as:

I ∝ 1/λ⁴

Because of this "fourth power" relationship, even a small difference in wavelength leads to a massive difference in scattering. For instance, red light has a wavelength approximately 1.8 times longer than blue light Science, The Human Eye and the Colourful World, p.169. When you plug this into the formula, you find that blue light is scattered nearly 10 times more efficiently than red light. This is why, on a clear day, the atmosphere effectively "steals" the blue light from the sun's direct beam and spreads it across the entire sky for our eyes to see.

The most spectacular application of this law is seen during sunrise and sunset. At these times, sunlight is hitting the Earth at a shallow angle, meaning it must travel through a much thicker layer of the atmosphere compared to when the sun is overhead at noon FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Solar Radiation, Heat Balance and Temperature, p.68. During this long journey, the shorter wavelengths (blue and violet) are scattered away almost entirely before the light reaches your eyes. The only colors that "survive" the long trip through the air are the ones that scatter the least: the long-wavelength reds and oranges.

Sources: Science, The Human Eye and the Colourful World, p.169; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Solar Radiation, Heat Balance and Temperature, p.68

7. Atmospheric Path Length and Sun's Color (exam-level)

To understand why the sun transforms from a brilliant white at noon to a deep crimson at dusk, we must look at the physics of Rayleigh Scattering. The Earth's atmosphere is a dense sea of nitrogen and oxygen molecules. These particles are significantly smaller than the wavelength of visible light. Because of their size, they interact selectively with light: they are far more effective at scattering shorter wavelengths (blue and violet) than longer wavelengths (red and orange) Science, Class X (NCERT 2025 ed.), Chapter 10, p.169. In fact, red light has a wavelength roughly 1.8 times larger than blue light, allowing it to pass through these particles with much less interference.

The defining factor for the color we see is the atmospheric path length. This refers to the total distance sunlight must travel through the Earth's gaseous envelope before reaching our eyes. This distance is not constant throughout the day:

Feature	Sun at Noon	Sun at Horizon (Sunrise/Sunset)
Path Length	Shortest (travels vertically)	Longest (travels obliquely)
Scattering	Minimal scattering of all colors	Maximum scattering of blue/violet light
Dominant Color	White/Yellowish-white	Red/Orange

At sunrise and sunset, the sun is near the horizon, and its light must pass through a much thicker layer of the atmosphere compared to when it is overhead Physical Geography by PMF IAS, Chapter 21, p.283. During this long journey, almost all the blue and violet light is scattered away in different directions, out of our line of sight. Only the "survivors"—the least-scattered longer wavelengths like red and orange—successfully complete the journey to our eyes. This phenomenon is also enhanced by the presence of fine dust and smoke particles in the lower atmosphere Fundamentals of Physical Geography, Class XI (NCERT 2025 ed.), Chapter 7, p.68.

While scattering determines the color, remember that atmospheric refraction handles the timing. Refraction allows us to see the sun about 2 minutes before the actual sunrise and 2 minutes after the actual sunset, often making the sun's disc appear slightly flattened Science, Class X (NCERT 2025 ed.), Chapter 10, p.168.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 10: The Human Eye and the Colourful World, p.168-169; Physical Geography by PMF IAS, Chapter 21: Horizontal Distribution of Temperature, p.283; Fundamentals of Physical Geography, Class XI (NCERT 2025 ed.), Chapter 7: Solar Radiation, Heat Balance and Temperature, p.68

8. Solving the Original PYQ (exam-level)

Now that you have mastered the principles of Rayleigh scattering and the visible light spectrum, you can see how these building blocks converge in this classic UPSC question. The critical concept here is the relationship between wavelength and scattering intensity. As you learned, shorter wavelengths (blue and violet) scatter easily when they hit atmospheric particles, while longer wavelengths (red and orange) are more resilient. As discussed in Science, class X (NCERT 2025 ed.), the atmosphere acts as a selective filter that is path-length dependent. During sunrise and sunset, sunlight must traverse a much thicker layer of the atmosphere compared to when the sun is overhead, forcing the light to encounter significantly more scattering particles before reaching your eyes.

To arrive at the correct answer, you must apply the logic of elimination based on the dominant physical process. Since the light has to travel a long distance through the dense lower atmosphere, the high-frequency blue light is "scattered out" of your line of sight long before it reaches you. Consequently, the light that successfully completes the journey is predominantly composed of the reddish-orange light because it is least scattered by the atmosphere, making (C) the correct choice. UPSC often uses distractor terms like "absorption" (Option B) or "reflection" (Option D) to confuse students; however, as noted in Physical Geography by PMF IAS, while these processes exist, they are not the primary cause of the sky's color shift. Similarly, Option (A) is a common trap suggesting the Sun itself changes its emission, whereas the Sun’s emission spectrum actually remains constant throughout the day.