Which one among the following has been producing/can produce light by a chemical change ?

1. Physical vs. Chemical Changes: The Basics (basic)

In the study of chemistry, we classify every transformation around us into two primary categories: physical and chemical changes. Understanding the distinction between them is like learning the grammar of the universe—it helps us understand why some things can be undone while others are changed forever.

A physical change is essentially a change in the outward appearance of a substance. It affects properties such as shape, size, and state (solid, liquid, or gas), but the fundamental nature of the substance remains untouched Science-Class VII, NCERT, Changes Around Us: Physical and Chemical, p.59. For example, when you melt an ice cube, it changes from solid to liquid, but it remains H₂O. No new substance is created, and most (though not all) physical changes are reversible. Think of it as a "makeover"—the person looks different, but they are still the same person inside.

A chemical change, however, is a deep, internal transformation. This occurs when a chemical reaction takes place, resulting in the formation of one or more new substances Science-Class VII, NCERT, Changes Around Us: Physical and Chemical, p.68. During this process, atoms are rearranged to create something with entirely different properties. Common everyday examples include combustion (burning), cooking, and rusting. Often, these changes are accompanied by signs like the evolution of a gas (bubbles), a change in color, or the release of energy as heat or light Science-Class VII, NCERT, Changes Around Us: Physical and Chemical, p.61.

Feature	Physical Change	Chemical Change
New Substance	No new substance is formed.	One or more new substances are formed.
Properties	Only physical properties (size, state) change.	Chemical properties change entirely.
Reversibility	Usually reversible (e.g., freezing water).	Usually irreversible (e.g., curdling milk).

Sources: Science-Class VII . NCERT(Revised ed 2025), Changes Around Us: Physical and Chemical, p.59; Science-Class VII . NCERT(Revised ed 2025), Changes Around Us: Physical and Chemical, p.61; Science-Class VII . NCERT(Revised ed 2025), Changes Around Us: Physical and Chemical, p.68

2. Sources of Light: Luminous and Non-Luminous Bodies (basic)

To master the chemistry of everyday phenomena, we must first understand how we perceive the world through light. In physics and chemistry, objects are classified based on their ability to generate light energy. This leads us to the distinction between Luminous and Non-Luminous bodies.

Luminous bodies are objects that emit their own light Science-Class VII . NCERT(Revised ed 2025), Light: Shadows and Reflections, p.165. They act as primary sources of light by converting one form of energy into light energy. For example, the Sun generates light through nuclear fusion (where hydrogen nuclei fuse to form helium), while a burning candle produces light through a chemical combustion reaction. Other examples include stars, fireflies, and glowing electric filaments.

Non-luminous bodies are objects that do not emit light of their own. We are able to see them only because they reflect light that falls on them from a luminous source Science-Class VII . NCERT(Revised ed 2025), Light: Shadows and Reflections, p.154. A common misconception is that the Moon is luminous because it shines brightly at night; however, the Moon is non-luminous. It simply reflects the sunlight hitting its surface back toward Earth. Similarly, planets like Mars and Venus, or everyday objects like chairs and mirrors, are non-luminous.

Feature	Luminous Bodies	Non-Luminous Bodies
Light Generation	Produce their own light energy.	Do not produce their own light.
Visibility	Visible due to the light they emit.	Visible only by reflecting light from others.
Examples	Sun, Stars, Electric Bulb, Fire.	Moon, Planets, Wood, Metals.

Sources: Science-Class VII . NCERT(Revised ed 2025), Light: Shadows and Reflections, p.154; Science-Class VII . NCERT(Revised ed 2025), Light: Shadows and Reflections, p.165

3. Electricity and Light: Incandescence and Discharge (intermediate)

To understand how we generate light using electricity, we must first understand the heating effect of electric current. When an electric current flows through a conductor (like a wire or a filament), it encounters resistance. This resistance causes the electrical energy to dissipate as heat, a phenomenon known as Joule heating Science, Class X (NCERT 2025 ed.), Electricity, p.188. While this is often an unwanted loss of energy in gadgets like fans, we purposefully harness it in devices like electric irons and heaters Science, Class VIII, NCERT (Revised ed 2025), Electricity: Magnetic and Heating Effects, p.53. When this heat becomes extreme, it leads to Incandescence — the process where a solid material becomes so hot that it begins to emit visible light.

In a traditional electric bulb, we use a tungsten filament because of its remarkably high melting point of 3380°C. The goal is to retain as much heat as possible within the filament so it glows brightly without melting Science, Class X (NCERT 2025 ed.), Electricity, p.190. To prevent the filament from burning up (oxidizing) at these temperatures, bulbs are filled with chemically inactive gases like Nitrogen or Argon. In contrast, Gas Discharge (seen in neon signs or lightning) produces light by passing electricity through a gas, causing the gas atoms to become 'excited' and release energy as light. Unlike the Sun, which generates light through nuclear fusion (a transformation of matter), or the Moon, which simply reflects light, these electrical methods are physical processes where energy changes form but the identity of the atoms remains the same.

Mechanism	How it Works	Everyday Example
Incandescence	Heat-induced glow of a solid filament.	Standard filament bulb, Toaster coils.
Gas Discharge	Electricity exciting gas atoms into plasma.	Neon signs, Lightning, Fluorescent tubes.

Sources: Science, Class X (NCERT 2025 ed.), Electricity, p.188, 190; Science, Class VIII, NCERT (Revised ed 2025), Electricity: Magnetic and Heating Effects, p.53

4. Atmospheric Electricity: Lightning and Thunder (intermediate)

To understand lightning and thunder, we must first look at the sky as a massive laboratory of static electricity. This phenomenon begins within towering Cumulonimbus clouds, which can extend vertically for over 20,000 feet Certificate Physical and Human Geography, Climate, p.138. Inside these clouds, violent updrafts and downdrafts cause ice particles and water droplets to rub against one another. Much like rubbing your feet on a carpet, this friction creates a charge separation: the lighter ice particles carry a positive charge to the upper reaches of the cloud, while the heavier droplets carry a negative charge to the base Science Class VIII NCERT, Pressure, Winds, Storms, and Cyclones, p.91.

When the electrical potential difference between these layers becomes staggering—often between 10⁹ to 10¹⁰ volts—the air's natural resistance breaks down. A massive surge of current (up to 1,000,000 amperes) flows between the charges Physical Geography by PMF IAS, Thunderstorm, p.349. This discharge is Lightning. Though we see it as light, it is fundamentally a physical movement of electrons (plasma) rather than a primary chemical reaction. However, this discharge is so energetic that it briefly superheats the air to temperatures between 15,000°C and 30,000°C—actually hotter than the surface of the Sun!

The sound we call Thunder is a direct result of this extreme heat. When the air is heated so abruptly, it expands with explosive force, sending a sonic shock wave through the atmosphere Geography of India, Climate of India, p.29. If you are very far from the strike, you might see the flash but not hear the sound; this is colloquially known as heat lightning. From a chemistry perspective, while the discharge itself is physical, the heat is so intense that it forces nitrogen and oxygen in the air to react, forming nitrogen oxides—a natural way the Earth "fixes" nitrogen into the soil.

Phenomenon	Primary Cause	Nature of Process
Lightning	Electrical discharge between charge-separated layers.	Physical (Electrostatic)
Thunder	Rapid expansion of air due to extreme heating.	Physical (Acoustic/Mechanical)

Sources: Certificate Physical and Human Geography, Climate, p.138; Science Class VIII NCERT, Pressure, Winds, Storms, and Cyclones, p.91; Physical Geography by PMF IAS, Thunderstorm, p.349; Geography of India, Climate of India, p.29

5. Nuclear Fusion: The Engine of the Sun (exam-level)

At the heart of every star, including our Sun, lies a process far more powerful than any chemical fire we see on Earth: Nuclear Fusion. Unlike common chemical reactions that involve the swapping of electrons between atoms, fusion involves the literal merging of atomic nuclei. In the Sun, the primary reaction is the fusion of hydrogen atoms into helium. This process requires a staggering temperature of several million degrees Celsius and immense pressure to overcome the natural electrical repulsion between protons Physical Geography by PMF IAS, The Universe, p.9.

Why does this release so much energy? When two light nuclei fuse, the mass of the resulting single nucleus is slightly less than the sum of the original masses. This "missing mass" is converted into pure energy according to Einstein’s famous equation, E = mc². In the Sun, this energy creates an outward pressure that perfectly balances the inward pull of gravity, keeping the star stable for billions of years Physical Geography by PMF IAS, The Universe, p.11. On Earth, we lack the massive gravitational force necessary to create these conditions naturally, which is why fusion does not occur in Earth’s core Physical Geography by PMF IAS, Earths Interior, p.59.

Nuclear fusion is also the "factory" for all elements in the universe. While the Sun currently fuses hydrogen, older and more massive stars (like Red Giants) reach temperatures high enough to fuse helium into heavier elements like carbon and oxygen Physical Geography by PMF IAS, The Universe, p.10. Truly heavy elements, such as gold and uranium, are only forged during the violent death of massive stars known as Supernovae Physical Geography by PMF IAS, The Universe, p.14. This means every atom in your body was once cooked inside a star!

Feature	Nuclear Fusion	Nuclear Fission
Process	Joining light nuclei (e.g., H + H → He)	Splitting heavy nuclei (e.g., Uranium)
Energy Yield	Extremely High (3-4x more than fission)	High
Conditions	Extreme temperature & pressure	Neutron bombardment

Sources: Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.9; Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.10; Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.11; Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.14; Physical Geography by PMF IAS, Earths Interior, p.59

6. Comparing Energy Sources: Nuclear vs. Physical vs. Chemical (exam-level)

To master applied chemistry, we must distinguish between how energy is released in the world around us. At the simplest level, we have physical changes, where a substance changes its form or state but no new substance is created Science-Class VII . NCERT(Revised ed 2025), Changes Around Us: Physical and Chemical, p.68. For instance, when an electric bulb glows, the tungsten filament becomes white-hot (incandescence) due to the flow of electrons, but it remains tungsten. Similarly, the Moon does not produce light; it merely reflects the Sun's light, which is a purely physical phenomenon of light bouncing off a surface.

Moving deeper, chemical changes involve the breaking and forming of bonds between atoms to create entirely new substances, such as during combustion or photosynthesis. In photosynthesis, plants capture light energy and convert it into chemical energy stored in carbohydrates like glucose Environment and Ecology, Majid Hussain, BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.15. However, the most powerful energy source is nuclear fusion. Unlike chemical reactions that swap electrons, fusion transforms the very identity of the atom. In the Sun, hydrogen atoms fuse to form helium (H + H → He) under extreme temperature and pressure, releasing gargantuan amounts of energy Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.9.

While we often separate 'nuclear' and 'chemical' in advanced physics, in general science, we categorize the Sun's energy as a transformation of matter. This sets it apart from 'physical' light sources like lightning (an electrical discharge in plasma) or light bulbs (heat-induced glow). Understanding this hierarchy—from reflection to electron-sharing to atomic fusion—is crucial for identifying how different objects in our universe 'power up.'

Source Type	Mechanism	Example
Physical	Reflection, state change, or movement of electrons.	Moon (reflection), Electric Bulb (incandescence).
Chemical	Breaking/forming molecular bonds; new substances formed.	Burning wood, Photosynthesis (Light → Chemical energy).
Nuclear	Transformation of the atomic nucleus (Fusion/Fission).	The Sun (Hydrogen fusing into Helium).

Sources: Science-Class VII . NCERT(Revised ed 2025), Changes Around Us: Physical and Chemical, p.68; Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.15; Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.), The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.9

7. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamental differences between physical and chemical changes, this question tests your ability to apply those concepts to large-scale natural phenomena. In a chemical change, the internal identity of the substance is transformed to create a new product, often releasing energy in the process. While we typically think of "chemical" at a molecular level, in the context of General Science PYQs, the focus is on the source of energy—specifically whether the light is a byproduct of a fundamental transformation of matter or simply a change in state or position.

Applying this logic, the Sun (Option A) is the correct choice because it generates light through nuclear fusion. In this process, hydrogen nuclei fuse to form helium, representing a transformation of the very identity of the matter involved. Although modern physics distinguishes between "chemical" and "nuclear" reactions, in the broader classification used in many competitive exams, the Sun is the only option where the internal composition of the source changes to produce energy. By contrast, the Moon is a classic UPSC trap; it merely reflects light, which is a physical phenomenon involving no change in the Moon's substance. Similarly, the Electric bulb relies on incandescence (heating a filament) or gas discharge, both of which are physical processes where the material remains chemically identical before and after emitting light.

Finally, Lightning and thunder are results of an atmospheric electrical discharge. While the intense heat of lightning can cause secondary chemical reactions in the air (like the formation of ozone), the primary mechanism of light production is the movement of electrons and the excitation of gas atoms into a plasma state, which is a physical transition. As an aspirant, your goal is to identify the primary driver of the light. Always look for the option where the "fuel" is fundamentally altered to release photons, making the Sun the most robust answer in this comparative set. Reference: NCERT Class 10 Science - Chemical Reactions and Equations.