Which one of the following processes explains the splitting of a beam of white light into its constituent colours?

1. Nature of Light and Laws of Reflection (basic)

Light is a form of energy that enables us to see the world around us. At its most fundamental level in geometrical optics, light is treated as traveling in straight lines, a concept known as the rectilinear propagation of light Science, Class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.135. When a beam of light strikes a surface, such as a mirror, it bounces back into the same medium. This phenomenon is called reflection. It is this reflected light that enters our eyes and allows us to perceive objects that do not emit their own light.

The behavior of light during reflection is governed by two critical Laws of Reflection. These laws are not just limited to flat mirrors; they are universal and apply to all types of reflecting surfaces, including curved or spherical ones Science, Class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.135. The laws are as follows:

• First Law: The angle of incidence (the angle between the incoming ray and the normal) is always equal to the angle of reflection (the angle between the reflected ray and the normal). Mathematically, ∠i = ∠r.
• Second Law: The incident ray, the normal to the mirror at the point of incidence, and the reflected ray, all lie in the same plane. Imagine them all sitting flat on a single sheet of paper.

When we look into a plane mirror, we see an image that has specific characteristics. The image is virtual (it cannot be projected onto a screen) and erect (upright). Crucially, the size of the image is exactly equal to the size of the object, and it appears to be at the same distance behind the mirror as the object is in front of it Science, Class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.135. This symmetry is a direct result of the laws of reflection acting on a flat surface.

Sources: Science, Class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.134-135

2. Refraction and the Refractive Index (basic)

When light travels from one transparent medium to another—say, from air into water or glass—it doesn't just pass straight through. Instead, it changes its direction at the boundary. This phenomenon is called refraction. At its heart, refraction happens because light travels at different speeds in different materials. Think of it like a shopping cart moving from a smooth pavement onto a patch of grass at an angle; as the front wheels hit the grass and slow down, the cart pivots. Similarly, light slows down and bends when it enters a more "resistant" medium Science, Class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.146.

To measure how much a medium slows down light, we use a value called the refractive index (n). It is a simple ratio: the speed of light in a vacuum (c) divided by the speed of light in that specific medium (v). The formula is n = c/v. Because light travels fastest in a vacuum (nearly 3 × 10⁸ m/s), the refractive index of any other material is always greater than 1.0. For example, the refractive index of water is about 1.33, while diamond has a much higher index of 2.42, meaning light travels significantly slower in a diamond Science, Class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.159.

Understanding how light bends is governed by Snell’s Law. It states that for a given pair of media, the ratio of the sine of the angle of incidence (i) to the sine of the angle of refraction (r) is constant. This constant is exactly the refractive index of the second medium relative to the first Science, Class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.148. A crucial distinction for your exams is between mass density and optical density. A material like turpentine has a lower mass density than water (it floats), but it is optically denser, meaning it has a higher refractive index and bends light more sharply Science, Class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.149.

Sources: Science, Class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.146; Science, Class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.148; Science, Class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.149; Science, Class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.159

3. Total Internal Reflection (TIR) and its Applications (intermediate)

To understand Total Internal Reflection (TIR), we must first look at how light behaves when it tries to escape a denser medium into a rarer one. When light travels from an optically denser medium (like glass or water) to an optically rarer medium (like air), it bends away from the normal. As we increase the angle of incidence, the angle of refraction also increases, bending further away from the normal until it eventually skims the surface of the boundary. The specific angle of incidence that results in an angle of refraction of 90° is called the Critical Angle.

If the incident angle is increased even slightly beyond this critical angle, the light can no longer pass through into the second medium. Instead, it is reflected entirely back into the denser medium, following the standard laws of reflection Science, class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.135. This unique phenomenon is what we call Total Internal Reflection. It is important to note that optical density is not the same as mass density; it refers to the medium's refractive index Science, class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.149. For TIR to occur, two strict conditions must be met:

TIR is the backbone of modern communication technology. Optical fibers use this principle to transmit data as light pulses over vast distances with minimal loss FUNDAMENTALS OF HUMAN GEOGRAPHY, CLASS XII (NCERT 2025 ed.), Transport and Communication, p.68. Because the light is "trapped" inside the fiber through continuous internal reflections, it can carry large quantities of data rapidly and securely. Other natural wonders like the brilliance of a diamond or the formation of a mirage in a hot desert are also classic examples of TIR at work.

Sources: Science, class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.135, 149; FUNDAMENTALS OF HUMAN GEOGRAPHY, CLASS XII (NCERT 2025 ed.), Transport and Communication, p.68

4. Scattering of Light and the Tyndall Effect (intermediate)

When light travels through a medium, it doesn't always move in a perfectly straight line. If it encounters obstacles like molecules of air, dust particles, or water droplets, those particles absorb the light energy and re-emit it in various directions. This phenomenon is known as the scattering of light. It is different from reflection because the light is spread out in many directions rather than bouncing off at a specific angle. The most famous demonstration of this is the Tyndall Effect, which you can see when a beam of sunlight enters a dusty room or passes through the canopy of a dense forest Science, Class X (NCERT 2025 ed.), Chapter 10, p.169. In these cases, the path of the light becomes visible because the suspended particles scatter the light toward our eyes.

The nature of scattering is governed by a critical rule: the size of the scattering particles determines the color of the scattered light. Our atmosphere is filled with nitrogen and oxygen molecules that are much smaller than the wavelength of visible light. These fine particles are highly effective at scattering shorter wavelengths (blue and violet) rather than longer wavelengths (red). In fact, red light has a wavelength about 1.8 times greater than blue light Science, Class X (NCERT 2025 ed.), Chapter 10, p.169. This is why the clear sky appears blue—the blue light is scattered in all directions and reaches our eyes from every part of the sky.

However, if the particles are large enough, the physics changes. Large particles, like the water droplets in a cloud or thick mist, do not selectively scatter just the blue light; they scatter all wavelengths of the visible spectrum almost equally. This is why clouds and thick fog appear white Science, Class X (NCERT 2025 ed.), Chapter 10, p.169. Interestingly, if Earth had no atmosphere, there would be no particles to scatter light at all. In such a scenario, the sky would appear completely black, just as it does to astronauts in space!

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

5. Diffraction and Polarization of Light (exam-level)

In our previous discussions, we treated light as a collection of rays traveling in straight lines. This works perfectly for mirrors and lenses, but as we dive deeper, we find that light has a hidden "wave" identity that reveals itself under specific conditions. When light encounters an extremely small opaque object or a narrow slit, it doesn't just stop or pass through; it has a tendency to bend around corners. This phenomenon is called Diffraction. As noted in Science, Class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.134, when the obstacle becomes very small, the straight-line treatment of optics fails, and we must recognize light as a wave to explain why shadows aren't perfectly sharp.

While diffraction proves that light is a wave, Polarization tells us exactly what kind of wave it is. Light is a transverse electromagnetic wave, meaning its electric and magnetic fields vibrate perpendicularly to the direction of travel Physical Geography by PMF IAS, Earths Magnetic Field, p.64. In ordinary light, these vibrations occur in all possible planes. Polarization is the process of filtering these vibrations so they occur in only one single plane. Think of it like trying to pass a vibrating jump rope through a picket fence; only the vibrations aligned with the gaps in the fence get through. This is why Polarized sunglasses are so effective—they block specific horizontal glare (vibrations) reflecting off surfaces like water or roads.

It is crucial for UPSC aspirants to distinguish between these two wave behaviors. Diffraction is a property shared by all waves (including sound), but Polarization is unique to transverse waves. Because sound is a longitudinal wave (vibrating parallel to its path), it cannot be polarized. Therefore, the fact that light can be polarized is the ultimate proof of its transverse nature.

Sources: Science, Class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.134; Physical Geography by PMF IAS, Earths Magnetic Field (Geomagnetic Field), p.64

6. Dispersion of White Light and the Glass Prism (intermediate)

When we look at a simple rectangular glass slab, light enters and exits parallel to its original path, merely shifting slightly to the side. However, a triangular glass prism changes the game entirely because its refracting surfaces are not parallel; they are inclined at an angle known as the angle of the prism Science, Chapter 10, p.165. When a beam of white light strikes this inclined surface, it doesn't just bend—it breathes life into a vibrant array of colors. This phenomenon of splitting white light into its constituent colors is what we call dispersion.

Why does this happen? Though all colors of light travel at the same breakneck speed in a vacuum, they behave differently inside a medium like glass. White light is actually a composite of seven main colors—remembered by the acronym VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, and Red). Each of these colors has a different wavelength, and in glass, different wavelengths travel at different speeds. Because the refractive index of glass varies with the speed of light, each color undergoes a different degree of refraction (bending) Science, Chapter 10, p.167.

As these colors exit the prism, they emerge along distinct paths, forming a beautiful band of colors called a spectrum. It was Sir Isaac Newton who first used a glass prism to demonstrate that sunlight is made of these seven colors. He elegantly proved that the prism wasn't "painting" the light, but rather revealing the hidden components already present within it Science, Chapter 10, p.167.

Sources: Science, The Human Eye and the Colourful World, p.165; Science, The Human Eye and the Colourful World, p.167

7. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamentals of light and its interaction with different media, this question serves as the perfect application of those building blocks. As you learned in Science, class X (NCERT), white light is not a single entity but a composite of different wavelengths. The core concept here is differential refraction: when white light enters a medium like a glass prism, each constituent color travels at a different speed. This causes them to bend at different angles—red bending the least and violet the most—resulting in the distinct band of colors we call a spectrum. The specific term for this beautiful separation is Dispersion.

To arrive at the correct answer, your reasoning should follow the path of the light beam itself. Ask yourself: is the light bouncing back, or is it passing through and changing? Since the light is "splitting" as it moves through a medium, you are looking for a phenomenon related to refraction. While refraction is the underlying mechanism, the resultant separation into constituent colors is defined specifically as Dispersion. This distinction is a classic UPSC requirement—identifying the most precise scientific term for a specific outcome.

UPSC often includes related optical phenomena as traps to test the depth of your conceptual clarity. Reflection is easily eliminated as it involves light bouncing off a surface without any change in its constituent composition. Diffraction is a common distractor; it refers to the bending of light around corners or through narrow apertures, not its separation into colors. Finally, Polarization refers to the process of restricting light waves to a single plane of vibration, which has no bearing on color splitting. By understanding that only Dispersion accounts for the wavelength-dependent separation of white light, you can navigate these distractors with ease.