Which one among the following colours has the highest wavelength ?

1. The Electromagnetic Spectrum Overview (basic)

To understand optics, we must first understand the Electromagnetic (EM) Spectrum. Think of it as a vast family of energy waves that travel through space. These waves are transverse in nature—meaning they vibrate perpendicular to the direction they travel, similar to the ripples you see when you toss a stone into a pond Physical Geography by PMF IAS, Earth's Interior, p.60. The spectrum is arranged based on two main characteristics: wavelength (the distance between two peaks) and frequency (how many peaks pass a point in a second).

There is a fundamental inverse relationship you must master: as wavelength decreases, the frequency and energy of the wave increase. At one extreme, we have Radio waves, which have the longest wavelengths—some even larger than our planet Physical Geography by PMF IAS, Earth's Atmosphere, p.279. At the other extreme are Gamma rays, which have the shortest wavelengths and the highest energy. In between, we find microwaves, infrared, visible light, ultraviolet, and X-rays.

The Visible Light Spectrum is the tiny slice of this range that human eyes can actually detect, spanning from approximately 400 nm to 700 nm Science, Class X (NCERT 2025 ed.), The Human Eye and the Colourful World, p.169. Within this visible band, Red light has the longest wavelength (and lowest energy), while Violet light has the shortest wavelength (and highest energy). Understanding this order is crucial because the way light interacts with matter—like the atmosphere or a glass lens—depends heavily on its wavelength.

Property	Red End (Long Wavelength)	Violet End (Short Wavelength)
Wavelength	High (~700 nm)	Low (~400 nm)
Frequency/Energy	Low	High

Sources: Physical Geography by PMF IAS, Earth's Interior, p.60; Physical Geography by PMF IAS, Earth's Atmosphere, p.279; Science, Class X (NCERT 2025 ed.), The Human Eye and the Colourful World, p.169

2. Wave Properties: Wavelength, Frequency, and Energy (basic)

To understand optics, we must first understand the nature of the wave itself. Imagine a wave as a traveling ripple. The highest point of this ripple is the crest, and the lowest is the trough. The wavelength is defined as the horizontal distance between two successive crests, while the wave height is the vertical distance from the trough to the crest Physical Geography by PMF IAS, Tsunami, p.192. In the context of light, wavelength determines the "color" we perceive. For instance, the visible spectrum ranges from approximately 400 nm (violet) to 700 nm (red). Within this range, red light has a wavelength nearly 1.8 times greater than blue light Science, Class X (NCERT), The Human Eye and the Colourful World, p.169.

There is a fundamental relationship between wavelength and frequency (the number of waves passing a point in one second). Since the speed of light in a vacuum is constant (represented by c), wavelength and frequency are inversely proportional. This means that as the wavelength gets longer, the frequency must get lower. Mathematically, this is expressed as v = fλ, where v is the speed, f is frequency, and λ (lambda) is wavelength. While the speed of light is nearly constant in air, it reduces significantly when entering denser media like glass or water, which is the root cause of refraction Science, Class X (NCERT), Light – Reflection and Refraction, p.148.

Finally, we must consider energy. In the electromagnetic spectrum, energy is directly proportional to frequency but inversely proportional to wavelength. Therefore, shorter waves (like violet light) pack more energy than longer waves (like red light). This is why high-frequency waves, such as microwaves, can cause significant energy interactions and are often absorbed by the Earth's atmosphere, whereas certain radio waves with longer wavelengths can be reflected by the ionosphere for long-distance communication Physical Geography by PMF IAS, Earths Atmosphere, p.278-279.

Property	Red Light	Violet Light
Wavelength	Longer (~700 nm)	Shorter (~400 nm)
Frequency	Lower	Higher
Energy	Lower	Higher

Sources: Physical Geography by PMF IAS, Tsunami, p.192; Science, Class X (NCERT), The Human Eye and the Colourful World, p.169; Science, Class X (NCERT), Light – Reflection and Refraction, p.148; Physical Geography by PMF IAS, Earths Atmosphere, p.278-279

3. Refraction and Dispersion of White Light (intermediate)

When we think of light, we often perceive it as a single, uniform entity. However, white light is actually a composite of multiple wavelengths. To understand how it splits, we must first look at the triangular glass prism. Unlike a rectangular glass slab where the opposite faces are parallel (causing the light to emerge parallel to its original path), a prism has two triangular bases and three rectangular lateral surfaces that are inclined at an angle. This specific geometry, known as the Angle of the Prism, ensures that the light entering the prism is bent twice—once upon entry and once upon exit—leading to a significant deviation from its original path Science, Class X (NCERT 2025 ed.), Chapter 10, p.165.

The phenomenon of splitting white light into its component colors is called dispersion. This happens because light of different colors travels at different speeds when passing through a medium like glass. While all colors of light travel at the same speed in a vacuum (approximately 3 × 10⁸ m/s), they interact differently with the atoms of a transparent medium. Red light, which has the longest wavelength (roughly 625–750 nm), travels the fastest in glass and is therefore bent the least. In contrast, violet light has the shortest wavelength (roughly 380–400 nm), travels the slowest, and is bent the most Science, Class X (NCERT 2025 ed.), Chapter 10, p.167. This differential bending is why we see a distinct band of colors, or a spectrum, arranged in the order of VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, and Red).

To grasp the physics behind this, we look at the refractive index (n). The refractive index of a medium is defined as the ratio of the speed of light in a vacuum (c) to the speed of light in that medium (v): n = c/v Science, Class X (NCERT 2025 ed.), Chapter 9, p.149. Since violet light slows down more than red light in glass, the refractive index of glass is effectively higher for violet light than it is for red light. This fundamental property—that the refractive index of a material depends on the wavelength of light—is the root cause of all chromatic dispersion we observe in nature, from rainbows to the brilliance of a diamond.

Feature	Red Light	Violet Light
Wavelength	Longest (~700 nm)	Shortest (~400 nm)
Speed in Glass	Highest	Lowest
Bending (Deviation)	Least	Most

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; Science, Class X (NCERT 2025 ed.), Chapter 9: Light – Reflection and Refraction, p.149

4. Scattering of Light and Rayleigh's Law (intermediate)

When we look at the world, we often see light that hasn't come to us in a straight line from a source, but has been redirected by the atmosphere. This phenomenon is known as Scattering of Light. At its most fundamental level, scattering occurs when light waves encounter small particles—such as gas molecules, dust, or water droplets—which absorb the energy and then re-emit it in various directions. The way this light behaves depends heavily on two factors: the size of the particle and the wavelength of the light.

In the visible spectrum, light consists of colors ranging from Violet (shortest wavelength, ~400 nm) to Red (longest wavelength, ~700 nm). Lord Rayleigh discovered a specific rule for very small particles (like Nitrogen and Oxygen molecules in our air). According to Rayleigh’s Law of Scattering, the intensity of scattered light is inversely proportional to the fourth power of its wavelength (Intensity ∝ 1/λ⁴). This means that shorter wavelengths are scattered far more efficiently than longer ones. Since blue light has a shorter wavelength than red light, it is scattered about 10 times more strongly, which is why a clear sky appears blue Science, Class X, Chapter 10, p.169.

However, the size of the "obstructing" particle changes the game entirely. If the particle (like a dust grain or water droplet) is much larger than the wavelength of light, Rayleigh's Law no longer applies. Instead, all wavelengths are scattered almost equally, often resulting in white light. This is why clouds, which are made of large water droplets, appear white rather than blue Science, Class X, Chapter 10, p.169. Furthermore, if the particle is significantly larger than the wavelength, we observe simple reflection rather than scattering Physical Geography by PMF IAS, Horizontal Distribution of Temperature, p.283.

Particle Size	Type of Scattering	Visual Result
Very Small (Gas molecules)	Rayleigh Scattering	Blue sky; Short wavelengths scatter most.
Large (Water droplets/Clouds)	Non-selective Scattering	White clouds; All wavelengths scatter equally.
Colloidal (Dust/Mist)	Tyndall Effect	Visible beam of light (e.g., in a forest).

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

5. Atmospheric Phenomena and Applications (exam-level)

To understand how we perceive the world around us, we must first look at the Visible Light Spectrum. White light is composed of seven constituent colors, often remembered by the acronym VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, and Red). These colors differ primarily in their wavelength: Red light sits at the long-wavelength end (~700 nm), while Violet has the shortest wavelength (~400 nm) Science, Class X, Chapter 10, p. 167. This physical difference dictates how light interacts with our atmosphere through two primary phenomena: Scattering and Refraction.

Atmospheric Scattering occurs when sunlight strikes molecules and small particles in the air. According to Rayleigh scattering, shorter wavelengths (blue/violet) are scattered much more strongly than longer wavelengths (red). This is why the clear sky appears blue. However, at very high altitudes where the atmosphere is thin, scattering is negligible, causing the sky to appear dark or black to astronauts and high-altitude pilots Science, Class X, Chapter 10, p. 169. Because Red light is least scattered by fog or smoke due to its long wavelength, it can travel longer distances without being dispersed, which is why it is universally used for danger signals.

Atmospheric Refraction is the bending of light as it passes through layers of air with varying densities. Since the air near the Earth's surface is denser than the air higher up, sunlight traveling toward Earth is continuously bent toward the 'normal.' This creates a fascinating optical illusion: 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 set (Delayed Sunset) Science, Class X, Chapter 10, p. 168. This phenomenon also causes the apparent flattening of the Sun's disc during these times.

Phenomenon	Cause	Observable Effect
Scattering	Interaction with small particles	Blue sky, Red sunset, Red danger signals
Refraction	Varying atmospheric density	Advanced sunrise, Delayed sunset, Twinkling stars

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.168; Science, Class X (NCERT 2025 ed.), Chapter 10: The Human Eye and the Colourful World, p.169

6. The Visible Spectrum (VIBGYOR) Characteristics (basic)

When white light passes through a prism, it splits into a beautiful band of colors known as the Visible Spectrum. This happens because different colors of light bend at slightly different angles as they pass through a medium. This sequence of colors is universally remembered by the acronym VIBGYOR: Violet, Indigo, Blue, Green, Yellow, Orange, and Red Science, Class X (NCERT 2025 ed.), Chapter 10, p.167. Each of these colors corresponds to a specific wavelength of electromagnetic radiation, ranging approximately from 400 nm (violet) to 700 nm (red).

It is crucial to understand the inverse relationship between wavelength and energy. As we move from Red to Violet, the wavelength decreases while the frequency and energy increase. For instance, Red light sits at the long-wavelength end (approx. 700 nm) and has the lowest energy, whereas Violet light has the shortest wavelength (approx. 400 nm) and the highest energy. Interestingly, the wavelength of Red light is about 1.8 times greater than that of Blue light Science, Class X (NCERT 2025 ed.), Chapter 10, p.169. This physical difference explains why colors behave differently when interacting with particles in our atmosphere.

Characteristic	Red End	Violet End
Wavelength	Longest (~700 nm)	Shortest (~400 nm)
Energy / Frequency	Lowest	Highest
Atmospheric Scattering	Least scattered	Most scattered

Sources: Science, Class X (NCERT 2025 ed.), Chapter 10: The Human Eye and the Colourful World, p.167, 169

7. Solving the Original PYQ (exam-level)

This question directly applies your understanding of the visible light spectrum and how electromagnetic radiation is categorized. As you have learned in Science, class X (NCERT 2025 ed.), white light is not a single entity but a mixture of colors that disperse into the VIBGYOR sequence. The fundamental concept to bridge here is the inverse relationship between energy and wavelength: as you move from the violet end to the red end of the spectrum, the energy of the photons decreases while the wavelength increases.

To arrive at the correct answer, you should mentally map the VIBGYOR acronym to its numerical ranges. Violet occupies the short-wavelength, high-energy side (around 380–400 nm), while Red sits at the opposite extreme, possessing the highest wavelength (approximately 625–750 nm). By recognizing that Red is at the far end of the visible range, you can confidently conclude that its waves are the most "stretched out," making (D) Red the correct choice. This physical property is why red light is used in danger signals; its long wavelength allows it to travel further through fog or smoke with minimal scattering.

UPSC frequently uses the other options as conceptual traps. Violet is the most common distractor; students often confuse "highest wavelength" with "highest frequency" or "highest energy," both of which would point to Violet. Green and Yellow serve to test your precise knowledge of the spectrum's order. Remember, in the visible range, Red is always the longest and Violet is always the shortest. If you can anchor those two extremes in your mind, you will avoid the confusion between energy-based and distance-based properties of light.