Rays of light get refracted while passing from air to glass because

1. Fundamental Nature of Light (basic)

To understand optics, we must first ask: What exactly is light? At its simplest level, light is a form of energy that provides us with the sensation of sight. In the broader context of physics and geography, light is part of the electromagnetic spectrum. For instance, the Earth receives energy from the Sun (insolation) primarily as short-wave radiation, which includes visible light and ultraviolet rays Physical Geography by PMF IAS, Horizontal Distribution of Temperature, p.282. This energy travels through the vacuum of space at a staggering speed of approximately 3.0 × 10⁸ m/s.

Historically, scientists grappled with the true nature of light. Is it a wave, like a ripple in a pond, or a stream of tiny particles? The answer, as it turns out, is both. This is known as the Dual Nature of Light. While phenomena like diffraction suggest light is a wave, its interaction with matter often suggests it behaves like a stream of particles. Modern Quantum Theory reconciles these views, stating that light is neither purely a wave nor purely a particle, but possesses characteristics of both Science, Class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.134.

In our current study of Geometrical Optics, we primarily use the concept of rectilinear propagation. This is the observation that light appears to travel in straight lines Science, Class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.158. By representing light as "rays"—straight lines with arrows indicating direction—we can simplify complex behaviors to study how mirrors and lenses form images. This approach is highly effective for explaining everyday phenomena, from the reflection in your dressing mirror to the blue color of the sky caused by the scattering of light by atmospheric particles Science, Class X (NCERT 2025 ed.), The Human Eye and the Colourful World, p.169.

Sources: Physical Geography by PMF IAS, Horizontal Distribution of Temperature, p.282; Science, Class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.134; Science, Class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.158; Science, Class X (NCERT 2025 ed.), The Human Eye and the Colourful World, p.169

2. Laws of Reflection and Mirrors (basic)

At its simplest level, reflection is the phenomenon where light rays strike a surface and bounce back into the same medium. This isn't a random occurrence; it follows two fundamental rules known as the Laws of Reflection. First, the angle of incidence (∠i)—the angle between the incoming ray and the perpendicular 'normal' line—is always equal to the angle of reflection (∠r). Second, the incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane. While we often visualize this with flat mirrors, it is vital to remember that these laws are universal; they apply to all reflecting surfaces, including curved or spherical mirrors Science, Class X (NCERT 2025 ed.), Chapter 9, p.139.

When light reflects, it forms an image, which can be categorized as either real or virtual. A real image is formed when light rays actually intersect at a point after reflection; these can be projected onto a screen. Conversely, a virtual image is formed when rays only appear to diverge from a point behind the mirror, such as the image you see of yourself in a dressing mirror every morning Science, Class X (NCERT 2025 ed.), Chapter 9, p.137. In plane mirrors, the image is always virtual, erect, and laterally inverted (left appears as right).

To quantify these images, we use magnification (m), which is the ratio of the height of the image to the height of the object. This value acts as a diagnostic tool: a negative sign in magnification tells us the image is real and inverted, while a positive sign indicates the image is virtual and erect Science, Class X (NCERT 2025 ed.), Chapter 9, p.143.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 9: Light – Reflection and Refraction, p.137, 139, 143

3. Scattering and Dispersion of Light (intermediate)

When we look at a beam of white light, it appears simple, but it is actually a complex mixture of different wavelengths. Dispersion is the phenomenon where white light splits into its constituent colors (VIBGYOR) when passing through a transparent medium like a glass prism. This happens because different colors of light travel at different speeds in a medium, even though they all travel at the same speed in a vacuum. In a glass prism, red light, which has a longer wavelength, travels the fastest and thus bends the least. Conversely, violet light, with a shorter wavelength, travels the slowest and bends the most Science, Class X (NCERT 2025 ed.), The Human Eye and the Colourful World, p.167. This differential bending creates a spectrum, a discovery famously first demonstrated by Isaac Newton.

While dispersion is about the "splitting" of light through a medium, Scattering is the "spreading" or deflection of light in various directions when it strikes particles like dust, gas molecules, or water droplets. This is known as the Tyndall Effect. The nature of scattering depends heavily on the size of the obstructing particle. Very fine particles in the atmosphere (like gas molecules) are more effective at scattering shorter wavelengths, such as blue light, which is why the clear sky appears blue. If the particles are larger (like water droplets in a cloud), they scatter all wavelengths of light nearly equally, making the light appear white Science, Class X (NCERT 2025 ed.), The Human Eye and the Colourful World, p.169.

Interestingly, the relationship between wavelength and particle size determines whether scattering or reflection occurs. If the wavelength of light is much larger than the particle, scattering dominates; however, if the wavelength is smaller than the particle, simple reflection takes place Physical Geography by PMF IAS, Horizontal Distribution of Temperature, p.283. This interplay is why the atmosphere filters and transforms sunlight before it reaches us.

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

4. Total Internal Reflection (TIR) (intermediate)

When light travels from an optically denser medium (like glass or water) to an optically rarer medium (like air), it speeds up and bends away from the normal Science, Class X, Chapter 9, p.149. As you increase the angle of incidence (the angle at which the light hits the boundary), the refracted ray bends further and further toward the surface. Eventually, you reach a specific threshold called the Critical Angle. At this precise angle, the refracted light ray doesn't escape into the air; it skims right along the boundary at 90° to the normal.

If you increase the angle of incidence even slightly beyond this critical angle, refraction becomes impossible. Instead of passing through, the light is completely reflected back into the original denser medium. This phenomenon is known as Total Internal Reflection (TIR). Unlike reflection from a standard silvered mirror, which always absorbs a small fraction of light, TIR is essentially 100% efficient, meaning no light energy is lost to the second medium. This makes it incredibly useful for technology.

One of the most transformative applications of TIR is in Optical Fiber Cables. These cables consist of a glass or plastic core through which light pulses travel. Because the light hits the walls of the fiber at angles greater than the critical angle, it stays trapped inside, bouncing along the length of the cable for miles. This allows for the rapid, secure, and high-capacity transmission of data that powers our modern internet Fundamentals of Human Geography, Class XII, Chapter 8, p.68.

Sources: Science, Class X, Light – Reflection and Refraction, p.149; Fundamentals of Human Geography, Class XII, Transport and Communication, p.68

5. Understanding Refraction and Snell's Law (intermediate)

When we observe a pencil partially immersed in water, it appears to be broken at the interface. This isn't an optical illusion, but a physical phenomenon called refraction. Refraction is the change in the direction of light as it passes from one transparent medium to another Science, Class X (NCERT 2025 ed.), Chapter 9, p.146. This happens because light travels at different speeds in different materials. While light cruises at its maximum speed of approximately 3.0 × 10⁸ m/s in a vacuum, it slows down significantly in denser materials like glass (about 2.0 × 10⁸ m/s) or water. Imagine a marching band moving from a paved road into a muddy field at an angle; the rows that hit the mud first slow down, causing the entire column to pivot. That is exactly how light waves behave.

To quantify how much light slows down, we use the refractive index (n). It is the ratio of the speed of light in a vacuum (c) to the speed of light in the medium (v). A higher refractive index means the medium is optically denser and light travels slower within it Science, Class X (NCERT 2025 ed.), Chapter 9, p.148. For example, glass has a refractive index of approximately 1.5, whereas air is very close to 1.0. When light moves from a medium with a lower refractive index (like air) to one with a higher index (like glass), it bends toward the normal. Conversely, when it speeds up moving into a rarer medium, it bends away from the normal.

The mathematical heart of this process is Snell’s Law. It states that for a given pair of media and a specific color of light, the ratio of the sine of the angle of incidence (i) to the sine of the angle of refraction (r) is a constant Science, Class X (NCERT 2025 ed.), Chapter 9, p.148. This constant is the refractive index of the second medium relative to the first. Additionally, the laws of refraction remind us that the incident ray, the refracted ray, and the normal at the point of incidence all lie in the same plane.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 9: Light – Reflection and Refraction, p.146; Science, Class X (NCERT 2025 ed.), Chapter 9: Light – Reflection and Refraction, p.148

6. Refractive Index and Optical Density (exam-level)

When light travels from one transparent medium to another, it doesn't just pass through; it changes its speed. This change in speed is the fundamental reason why light bends, a phenomenon we call refraction. To quantify how much a medium slows down light, we use a constant called the Refractive Index. The absolute refractive index (nₘ) of a medium is defined as the ratio of the speed of light in a vacuum (c) to the speed of light in that specific medium (v). Mathematically, it is expressed as nₘ = c/v Science, Class X (NCERT 2025 ed.), Chapter 9: Light – Reflection and Refraction, p.148. Since the speed of light is highest in a vacuum (approximately 3.0 × 10⁸ m/s), the refractive index of any other material is always greater than 1.

In the study of optics, we often use the terms 'optically rarer' and 'optically denser' to compare two media. A medium with a higher refractive index is considered optically denser, meaning light travels through it more slowly. Conversely, a medium with a lower refractive index is optically rarer, and light travels faster through it Science, Class X (NCERT 2025 ed.), Chapter 9: Light – Reflection and Refraction, p.149. For example, light travels slower in glass (n ≈ 1.5) than in water (n ≈ 1.33), making glass optically denser than water.

It is vital to understand that optical density is NOT the same as mass density. While mass density is the mass per unit volume (kg/m³), optical density is specifically a measure of a medium's ability to refract light. A classic example is kerosene: it has a lower mass density than water (it floats on water), yet it has a higher refractive index (1.44) than water (1.33). This means kerosene is optically denser than water, even though it is physically lighter Science, Class X (NCERT 2025 ed.), Chapter 9: Light – Reflection and Refraction, p.149.

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

7. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamental properties of light, this question brings those building blocks together to test your understanding of refraction. You have learned that light behaves differently depending on the optical density of the medium it traverses. While terms like "refractive index" might seem like abstract numbers, they are actually direct measurements of how much a medium resists the flow of light. As you move from the conceptual phase to application, remember that refraction is not just an "event" that happens at a boundary; it is a physical consequence of a change in velocity.

To arrive at the correct answer, follow the logic of the refractive index. According to Science, class X (NCERT 2025 ed.), the refractive index is defined as the ratio of the speed of light in a vacuum to its speed in a specific medium. When light transitions from air (refractive index ≈ 1.0) to glass (refractive index ≈ 1.5), it is entering a more optically dense medium. This transition causes the light to slow down significantly—from approximately 3.0 × 10⁸ m/s to 2.0 × 10⁸ m/s. This change in speed is the fundamental cause of the bending toward the normal. Therefore, (D) speed of light in glass is less than the speed of light in air is the precise scientific explanation for the phenomenon.

UPSC often includes "distractor" options that sound plausible but lack scientific rigor. Option (A) is a common trap; while glass has a higher mass density than air, refraction is governed by optical density, making (A) a secondary characteristic rather than the primary cause. Option (B) is factually incorrect as light can indeed reflect off glass (think of a window pane), and Option (C) describes absorption, which affects intensity but not the direction of travel. In your exams, always look for the kinetic reason (speed) behind the geometric result (bending) to find the right answer.