A ray of light when refracted suffers change in velocity. In this context, which one among the following statements is correct ?

1. Nature of Light and Propagation (basic)

To understand optics, we must first look at the unique nature of light itself. For centuries, scientists debated whether light was a stream of particles or a wave. Today, we understand through modern quantum theory that light possesses a dual nature—it exhibits both wave-like and particle-like properties depending on the experiment Science, Class X (NCERT 2025 ed.), Chapter 9, p.134. In the study of Geometrical Optics, we primarily treat light as rays that travel in straight lines, a concept known as the rectilinear propagation of light.

The most critical thing to know about the propagation of light is that its speed is not universal across all substances. While light travels at its maximum speed of approximately 3 × 10⁸ m/s in a vacuum, it slows down when it enters transparent materials like water or glass Science, Class X (NCERT 2025 ed.), Chapter 9, p.148. This variation in speed is the root cause of refraction—the bending of light. We use the term optical density to describe a medium's ability to slow down light; a medium that slows light down significantly is considered "optically denser" than one where light travels faster.

The relationship between these speeds is mathematically captured by the refractive index (n). It is defined as the ratio of the speed of light in a vacuum to the speed of light in that specific medium Science, Class X (NCERT 2025 ed.), Chapter 9, p.159. Therefore, a high refractive index indicates a low speed of light and high optical density. When a light ray transitions between two media of different densities, it changes direction: if it enters a denser medium (where it slows down), it bends towards the normal; if it enters a rarer medium (where it speeds up), it bends away from the normal.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 9: Light – Reflection and Refraction, p.134; Science, Class X (NCERT 2025 ed.), Chapter 9: Light – Reflection and Refraction, p.148; Science, Class X (NCERT 2025 ed.), Chapter 9: Light – Reflection and Refraction, p.159

2. The Phenomenon of Refraction (basic)

While reflection is about light bouncing back, refraction is about light entering a new medium and changing its path. Simply put, refraction is the bending of light as it passes obliquely from one transparent medium to another. This isn't just a random occurrence; it is a fundamental property of how light interacts with matter. As we explore in Science, Class X (NCERT 2025 ed.), Chapter 9, p.134, understanding this phenomenon is the key to explaining everything from the twinkling of stars to how a simple lens works.

The root cause of refraction is the change in the speed of light. Light travels at its maximum speed in a vacuum (approximately 3 × 10⁸ m/s) and slows down when it enters any other material. We measure a material's ability to slow down light using a term called optical density. It is important to distinguish this from mass density; optical density specifically refers to the medium's refractive power. A medium with a higher refractive index is considered optically denser, and light travels slower within it Science, Class X (NCERT 2025 ed.), Chapter 9, p.148.

The direction in which light bends depends entirely on whether it is speeding up or slowing down during the transition. You can master these rules using the table below:

Mathematically, this behavior follows the Laws of Refraction. The first law states that the incident ray, the refracted ray, and the normal all lie in the same plane. The second law, known as Snell’s Law, gives us a constant ratio between the angles of incidence (i) and refraction (r): sin i / sin r = constant. This constant is the refractive index of the second medium relative to the first Science, Class X (NCERT 2025 ed.), Chapter 9, p.148.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 9: Light – Reflection and Refraction, p.134; Science, Class X (NCERT 2025 ed.), Chapter 9: Light – Reflection and Refraction, p.147; Science, Class X (NCERT 2025 ed.), Chapter 9: Light – Reflection and Refraction, p.148; Science, Class X (NCERT 2025 ed.), Chapter 9: Light – Reflection and Refraction, p.149

3. Laws of Refraction and Snell's Law (intermediate)

When light travels from one transparent medium to another, it rarely continues in a straight line; it bends at the interface. This phenomenon is refraction, and it is governed by two fundamental laws. First, the incident ray, the refracted ray, and the normal to the interface at the point of incidence all lie in the same plane Science, Class X (NCERT 2025 ed.), Chapter 9, p.148. This ensures the path of light stays within a single geometric plane during the transition.

The second law, famously known as Snell’s Law of Refraction, provides the mathematical framework for this bending. It states that the ratio of the sine of the angle of incidence (i) to the sine of the angle of refraction (r) is a constant for a given pair of media and color of light: sin i / sin r = constant. This constant is known as the refractive index (n) of the second medium relative to the first Science, Class X (NCERT 2025 ed.), Chapter 9, p.148. Physically, this constant represents how much the speed of light changes as it moves between materials.

Refraction occurs because light travels at different speeds in different media. We use the term optical density to describe this; it is not the same as mass density. For example, kerosene has a higher refractive index (1.44) than water (1.33), meaning it is optically denser even though it floats on water Science, Class X (NCERT 2025 ed.), Chapter 9, p.149. When light enters an optically denser medium, its speed decreases and it bends toward the normal. Conversely, when it enters a rarer medium, it speeds up and bends away from the normal.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 9: Light – Reflection and Refraction, p.148; Science, Class X (NCERT 2025 ed.), Chapter 9: Light – Reflection and Refraction, p.149

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

5. Dispersion and Atmospheric Refraction (intermediate)

To understand why a simple beam of white light can transform into a beautiful rainbow, we must start with the Refractive Index. Light travels at different speeds in different media; it is fastest in a vacuum and slows down when entering a denser medium like glass or water. Science, Class X (NCERT 2025 ed.), Chapter 9: Light – Reflection and Refraction, p.148. However, there is a fascinating nuance: the refractive index of a medium is not the same for all colors. In a glass prism, Violet light travels the slowest and thus bends the most, while Red light travels the fastest (relative to other colors) and bends the least. This phenomenon, where white light splits into its constituent colors due to varying degrees of bending, is known as Dispersion. Science, Class X (NCERT 2025 ed.), Chapter 10: The Human Eye and the Colourful World, p.167.

While a rectangular glass slab emerges light parallel to its incident path, the unique triangular geometry of a prism causes the emergent rays to deviate significantly from their original direction. The band of colors produced—Violet, Indigo, Blue, Green, Yellow, Orange, and Red—is called a Spectrum. Isaac Newton was the first to demonstrate that this wasn't the prism 'coloring' the light, but rather revealing the light's internal components. Science, Class X (NCERT 2025 ed.), Chapter 10: The Human Eye and the Colourful World, p.167.

Beyond glass prisms, we see Atmospheric Refraction in our daily lives. The Earth's atmosphere is not uniform; its density and temperature change with altitude, creating layers with different refractive indices. As starlight enters the atmosphere, it undergoes continuous refraction. This causes the twinkling of stars (due to the shifting apparent position) and explains why we see the Sun about two minutes before the actual sunrise and two minutes after the actual sunset.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 9: Light – Reflection and Refraction, p.148; Science, Class X (NCERT 2025 ed.), Chapter 10: The Human Eye and the Colourful World, p.167

6. Optical Density and Velocity of Light (exam-level)

At its core, refraction is not just about light bending; it is a direct consequence of light changing its velocity as it moves from one material to another. While light travels at its maximum speed of approximately 3 × 10⁸ m s⁻¹ in a vacuum, it slows down significantly when it enters materials like water or glass Science, Class X (NCERT 2025 ed.), Chapter 9: Light – Reflection and Refraction, p. 148.

To quantify this phenomenon, we use the term optical density. It is vital to understand that optical density is distinct from the mass density (mass per unit volume) you studied in earlier grades. For instance, a substance might be lighter in weight (lower mass density) but still be more "optically dense" if it slows down light more effectively. In optics, we use the refractive index as the primary indicator of optical density: the higher the refractive index, the higher the optical density, and the slower light travels through that medium Science, Class X (NCERT 2025 ed.), Chapter 9: Light – Reflection and Refraction, p. 149.

When a light ray transitions between these media, its change in speed dictates its path. If light enters an optically denser medium (like moving from air to water), it slows down and bends towards the normal. Conversely, if it escapes into an optically rarer medium, it speeds up and bends away from the normal Science, Class X (NCERT 2025 ed.), Chapter 9: Light – Reflection and Refraction, p. 150. This relationship between velocity and optical density is what makes lenses and prisms work.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 9: Light – Reflection and Refraction, p.148; Science, Class X (NCERT 2025 ed.), Chapter 9: Light – Reflection and Refraction, p.149; Science, Class X (NCERT 2025 ed.), Chapter 9: Light – Reflection and Refraction, p.150

7. Solving the Original PYQ (exam-level)

This question brings together three fundamental pillars of optics you’ve just studied: optical density, the refractive index, and the velocity of light. As per Science, class X (NCERT 2025 ed.), refraction isn't just about light "bending"; it is the physical result of light changing speed as it crosses the boundary between materials with different optical properties. Remember the golden rule: optical density is a measure of a medium's ability to slow down light. Therefore, a medium with a higher refractive index is "optically denser," meaning light encounters more resistance, resulting in a lower velocity.

To arrive at the correct answer, follow the logic of the inverse relationship. When a ray moves from an optically rarer medium (like air) to an optically denser medium (like glass), it transitions from a medium where it travels faster to one where it travels slower. Since the speed of light is at its maximum in a vacuum and decreases as it enters more optically complex structures, moving into a "denser" environment logically leads to a decrease in velocity. This confirms that (C) Velocity decreases as the ray passes from a rarer to a denser medium is the only scientifically sound conclusion.

UPSC often uses "directional traps" in options (A) and (B) to test if you can keep the relationship straight under exam pressure—always double-check if the movement is rarer-to-denser or denser-to-rarer. Option (D) is a conceptual distractor; the nature of the medium is the primary determinant of light’s speed, as the refractive index is a property inherent to the material itself. By mastering these building blocks, you can avoid these common pitfalls and see through the phrasing of the question.