Which one of the following types of glass can cut off ultraviolet rays?

1. Glass as an Amorphous Solid (basic)

To understand glass, we must first look at the fundamental nature of matter. In a typical solid, the constituent particles are closely packed and held together by strong interparticle forces, giving them a fixed shape and volume Science, Class VIII, Particulate Nature of Matter, p.113. However, not all solids are organized in the same way. Most common solids, like iron or ice, are crystalline—meaning their atoms are arranged in a highly ordered, repeating geometric pattern. Glass is the great exception; it is an amorphous solid (from the Greek amorphos, meaning 'without form').

While the particles in glass are closely packed just like in a regular solid Science, Class VIII, Particulate Nature of Matter, p.113, they lack the long-range repeating order of crystals. Instead, their arrangement is more chaotic, similar to the disordered state found in liquids. Because of this unique internal structure, glass does not have a sharp melting point like urea or iron Science, Class VIII, Particulate Nature of Matter, p.103. Instead of turning instantly from a solid to a liquid at a specific temperature, glass gradually softens over a range of temperatures, which allows artisans to blow and mold it into various shapes.

Because glass shares some structural similarities with liquids but behaves like a solid, scientists often refer to it as a supercooled liquid. This means it is a substance that has cooled to a rigid state without crystallizing. This unique 'frozen liquid' state is what gives glass its characteristic transparency and its ability to refract light differently than other media like water or plastic Science, Class X, Light – Reflection and Refraction, p.145.

Feature	Crystalline Solid (e.g., Ice, Iron)	Amorphous Solid (e.g., Glass)
Particle Arrangement	Ordered, long-range repeating pattern.	Disordered, irregular arrangement.
Melting Point	Sharp and definite temperature.	Gradual softening over a range.
Common Name	True Solid	Pseudo-solid or Supercooled liquid

Sources: Science, Class VIII, NCERT (Revised ed 2025), Particulate Nature of Matter, p.113; Science, Class VIII, NCERT (Revised ed 2025), Particulate Nature of Matter, p.103; Science, Class X, NCERT (2025 ed.), Light – Reflection and Refraction, p.145

2. Soda-Lime Glass: The Everyday Standard (basic)

When we look through a window or drink from a glass jar, we are almost always interacting with Soda-Lime glass. While pure silica (SiO₂) could theoretically make glass on its own, it requires incredibly high temperatures—over 1700°C—to melt. To make glass production practical and affordable, we add specific chemical "fluxes" and "stabilizers." This creates the standard recipe used for about 90% of the world's glass products.

The name "Soda-Lime" tells you exactly what is inside. The recipe consists of three primary ingredients:

• Silica (Sand): The main structural component.
• Soda (Sodium Carbonate, Na₂CO₃): As noted in chemical applications, sodium carbonate is a fundamental ingredient in the glass industry Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.32. Its role is to act as a flux, lowering the melting point of silica to a more manageable 1000°C.
• Lime (Calcium Carbonate, CaCO₃): Adding soda alone makes the glass soluble in water (literally "water glass"). To prevent your glass from dissolving when it rains, we add Lime as a stabilizer. Interestingly, this is the same chemical compound that provides the shiny finish in whitewashing and constitutes the structure of marble Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.7.

While Soda-Lime glass is wonderful for its transparency and ease of shaping, it has limitations. It is sensitive to "thermal shock," meaning it can crack if subjected to sudden temperature changes (like pouring boiling water into a cold jar). Despite this, its low cost and chemical stability make it the undisputed king of everyday materials, from window panes to food packaging containers.

Sources: Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.32; Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.7

3. Understanding the Electromagnetic Spectrum: UV Radiation (intermediate)

In our journey through applied chemistry, understanding Ultraviolet (UV) radiation is vital because of its high energy and its ability to interact with matter at a molecular level. UV radiation occupies the portion of the electromagnetic spectrum between visible violet light and X-rays. Because UV rays have shorter wavelengths than visible light, they possess higher energy—enough to break chemical bonds and alter biological molecules like DNA. We generally categorize them into three types: UV-A (longest wavelength, least harmful), UV-B (highly biologically active), and UV-C (highest energy, but almost entirely absorbed by the atmosphere).

The biological impact of UV radiation, specifically UV-B, is a major concern in environmental chemistry. While the stratospheric ozone layer acts as a natural screen, any depletion allows more UV-B to reach the Earth's surface Environment and Ecology, Majid Hussain, Environmental Degradation and Management, p.12. In humans, prolonged exposure to UV-B doesn't just cause sun-burns; it acts on the immune system, making the body more susceptible to infectious diseases and decreasing the immune response to skin cancers Environment, Shankar IAS Academy, Ozone Depletion, p.267. It is also a leading cause of cataracts and damage to the cornea and lens of the eye Environment, Shankar IAS Academy, Ozone Depletion, p.271.

To protect ourselves from these high-energy waves, we use applied chemistry to create specialized materials. While ordinary soda-lime glass (used in windows) allows most UV radiation to pass through, Crookes glass is specifically engineered for eye protection. By incorporating Cerium Oxide (CeO₂) into the glass melt, the material gains the ability to strongly absorb ultraviolet radiation while remaining transparent to visible light. Similarly, in the pharmaceutical industry, amber-colored glass (created using iron and sulfur additives) is used to prevent UV rays from degrading light-sensitive medications.

Type of UV	Atmospheric Fate	Biological Impact
UV-A	Reaches surface easily	Contributes to skin aging
UV-B	Partially filtered by Ozone	Causes DNA mutations, cataracts, and immune suppression
UV-C	Completely absorbed by O₂ and O₃	Lethal but rarely reaches ground level

Sources: Environment, Shankar IAS Academy, Ozone Depletion, p.267; Environment, Shankar IAS Academy, Ozone Depletion, p.271; Environment and Ecology, Majid Hussain, Environmental Degradation and Management, p.12

4. Borosilicate Glass: Pyrex and Jena (intermediate)

When we think of glass, we often think of something fragile that shatters when exposed to sudden temperature changes. However, in the world of Applied Chemistry, we have developed specialized types of glass like Borosilicate glass—most commonly known by the brand names Pyrex and Jena—to solve this exact problem. While ordinary soda-lime glass is made primarily of silica, soda, and lime, borosilicate glass incorporates Boron Trioxide (B₂O₃) into the silica matrix. This chemical adjustment drastically reduces the material's coefficient of thermal expansion, meaning it doesn't expand or contract significantly when heated or cooled. This is why a Pyrex beaker can be moved from a flame to a cold surface without shattering from thermal shock.

The foundation of all glassmaking lies in minerals like Quartz (pure Silicon Dioxide) and Feldspar, which provides the necessary silicon and aluminum components Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.175. In borosilicate variants, the addition of boron creates a more robust atomic network. Beyond its thermal properties, glass is an exceptional insulator, with resistivity levels reaching as high as 10¹⁷ Ω m, making it safe for use in complex lab setups involving both heat and electrical components Science, Class X (NCERT 2025 ed.), Electricity, p.179.

It is important to distinguish between different high-performance glasses based on their additives:

• Pyrex/Jena (Borosilicate): Optimized for thermal resistance and chemical durability. Ideal for laboratory beakers and kitchen bakeware Science, Class VIII, The Amazing World of Solutes, Solvents, and Solutions, p.137.
• Soda-lime Glass: The "ordinary" glass used for windows and bottles; cheap but prone to thermal cracking.
• Specialized Optical Glass: While borosilicate is transparent, it does not inherently block UV light unless specifically treated with metal oxides like cerium or iron.

Feature	Soda-Lime Glass	Borosilicate Glass (Pyrex/Jena)
Main Additive	Sodium Carbonate (Soda)	Boron Trioxide (B₂O₃)
Thermal Shock	Low resistance (cracks easily)	High resistance (withstands heat)
Primary Use	Window panes, jars	Lab beakers, ovenware

Sources: Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.175; Science, Class X (NCERT 2025 ed.), Electricity, p.179; Science, Class VIII (NCERT 2025 ed.), The Amazing World of Solutes, Solvents, and Solutions, p.137

5. Optical Glass and Rare Earth Additives (exam-level)

When we think of glass, we often imagine a simple, transparent material. However, in the world of optical chemistry, glass is a sophisticated matrix designed to manipulate light. Most glass begins with silica (SiO₂), which is the primary component of quartz and sand Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.175. While standard soda-lime glass is excellent for windows, it allows a significant amount of ultraviolet (UV) radiation to pass through. To create specialized optical glass that protects our eyes or sensitive instruments, chemists introduce specific metal oxides into the molten silica mix.

The magic happens through the addition of Rare Earth elements and transition metals. For example, while many metal oxides like aluminium oxide or zinc oxide are added to glass to improve its chemical durability or thermal resistance Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.41, they do not inherently block UV rays. To achieve UV protection, a specific additive called cerium oxide (CeO₂) is used. When mixed with silica, it creates what is known as Crookes glass. The cerium atoms have a unique electron configuration that allows them to absorb high-energy UV photons while remaining transparent to visible light, effectively acting as a "chemical shield."

Glass Type	Key Additive	Primary Application
Soda-Lime	Sodium & Calcium Oxides	Windows, bottles, everyday glassware.
Borosilicate	Boron Trioxide (B₂O₃)	Laboratory equipment (Pyrex); heat resistant.
Crookes Glass	Cerium Oxide (CeO₂)	Sunglasses and optical lenses for UV protection.

It is important to distinguish between coloration and UV absorption. While amber glass (often used in medicine bottles) uses iron oxides to block light, Crookes glass is specifically engineered for high-end optical use because it can block harmful UV rays without significantly distorting the colors of the visible spectrum. This makes it the gold standard for protective eyewear, ensuring that the high-energy radiation from the sun doesn't reach the delicate tissues of the human eye.

Sources: Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.175; Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.41

6. Crookes Glass: The UV Shield (exam-level)

Glass has been a part of human civilization for millennia, evolving from ornamental beads and Roman bowls found in ancient trade sites like Pattanam to the highly specialized materials we use today History, class XI (Tamilnadu state board 2024 ed.), Evolution of Society in South India, p.71-74. While we often think of glass as a simple, transparent barrier, its chemistry can be precisely engineered. In everyday life, standard soda-lime glass (the kind used in window panes or drinking tumblers) is excellent for letting in visible light, but it is surprisingly transparent to many ultraviolet (UV) rays, which can cause long-term damage to the human eye.

Crookes glass is a specialized optical glass designed specifically to solve this problem. Developed by the scientist Sir William Crookes, this glass incorporates Cerium Oxide (CeO₂) into the silica mix. From a first-principles perspective, the inclusion of cerium ions changes how the glass interacts with the electromagnetic spectrum. While ordinary glass might be used in a lab to observe chemical reactions like the turning of lime water milky Science-Class VII, NCERT (Revised ed 2025), Changes Around Us: Physical and Chemical, p.60, Crookes glass acts as a selective filter. The cerium oxide molecules possess an electronic structure that allows them to absorb high-energy UV photons, effectively "cutting off" harmful radiation before it reaches the eye, all while remaining transparent to the visible light we need to see.

It is important to distinguish Crookes glass from other specialized glasses. For instance, borosilicate glass (often known by brands like Pyrex) is formulated with boron trioxide to resist thermal shock, making it ideal for lab equipment, but it does not inherently block UV light. Similarly, while we can see images in metal mirrors like the Aranmula Kannadi Science-Class VII, NCERT (Revised ed 2025), Light: Shadows and Reflections, p.162, these surfaces reflect light rather than filtering it. Crookes glass remains the gold standard for high-quality sunglasses and protective eyewear because it provides this critical "UV shield" without significantly distorting the colors of the world around us.

Sources: History, class XI (Tamilnadu state board 2024 ed.), Evolution of Society in South India, p.71-74; Science-Class VII, NCERT (Revised ed 2025), Changes Around Us: Physical and Chemical, p.60; Science-Class VII, NCERT (Revised ed 2025), Light: Shadows and Reflections, p.162

7. Solving the Original PYQ (exam-level)

This question perfectly bridges the gap between your study of glass composition and the practical application of optical properties. In our recent modules, we explored how the base structure of silica is modified by specific metal oxides to achieve functional variations. To arrive at the correct answer, you must apply the principle that the absorption of specific wavelengths depends on these chemical additives. Crookes glass is the correct choice because it is uniquely formulated with cerium oxide. This specific component acts as a chemical filter that strongly absorbs high-energy ultraviolet (UV) radiation while remaining transparent to visible light, making it the ideal material for protective eyewear.

In the UPSC context, it is vital to distinguish between optical protection and thermal resistance to avoid common traps. For instance, Pyrex glass and Jena glass are both borosilicates; while they are world-renowned for their ability to withstand sudden temperature changes, they do not inherently block UV rays. Similarly, Soda glass (the common soda-lime glass) is designed for general transparency in windows and bottles, allowing most UV light to pass right through. As a student, always look for the specific dopant mentioned in NCERT General Science—if the goal is UV cutoff, your reasoning should lead you directly to the cerium-infused Crookes glass.