Explanation: Nitrogen (N₂) constitutes the major portion of the Earth's atmosphere by volume — about 78%. It is a colorless, odorless, and inert gas that plays a vital role in maintaining atmospheric stability and supporting life through the nitrogen cycle. Oxygen makes up about 21% of the atmosphere and is essential for respiration and combustion. Other gases like argon (~0.93%) and carbon dioxide (~0.04%) are present in much smaller amounts.

Explanation: A rainbow is formed in the part of the sky opposite to the Sun. It occurs when sunlight is refracted, internally reflected, and dispersed by raindrops, splitting the light into its seven constituent colors. The observer must have the Sun behind them and rain in front to see the rainbow clearly.

Explanation: The onion is a modified underground stem called a bulb. It consists of a short, disc-like stem (the basal plate) surrounded by concentric fleshy leaf bases that store food. The roots emerge from the underside of the basal plate, and new shoots grow from the center. Thus, the onion bulb serves as a storage organ for nutrients and helps in vegetative propagation.

Explanation: Aurora Borealis, also known as the Northern Lights, is a natural light phenomenon visible mainly in the northern hemisphere, near the Arctic regions. It occurs when charged particles from the solar wind interact with gases like oxygen and nitrogen in the Earth's upper atmosphere (ionosphere). These collisions release energy in the form of colorful lights, usually green, pink, red, or violet. A similar phenomenon in the southern hemisphere is called the Aurora Australis or Southern Lights.

The Big Bang Theory explains the origin of the Universe. According to this theory, the universe began about 13.8 billion years ago from an extremely hot and dense state that expanded rapidly in all directions. This expansion continues even today, leading to the formation of galaxies, stars, planets, and other cosmic structures. The discovery of the cosmic microwave background radiation provides strong evidence supporting the Big Bang Theory.

Other options (for context)

• Origin of Sun: Explained by the Nebular Hypothesis, not the Big Bang Theory.
• Laws of Chemistry: Studied under physical and inorganic chemistry, unrelated to cosmology.
• Laws of History: Concern human civilizations, not the universe’s formation.

Explanation: The transmission of light through optical fibres is based on the principle of Total Internal Reflection (TIR). When light travels from a denser medium (core of the fibre) to a rarer medium (cladding) and strikes the interface at an angle greater than the critical angle, it gets completely reflected back into the core instead of refracting out. This continuous internal reflection allows light to travel long distances through the fibre with minimal loss, even when the fibre is bent. Optical fibres are widely used in telecommunication, internet data transfer, and medical endoscopy.

A rocket functions based on Newton’s Third Law of Motion, which states that “for every action, there is an equal and opposite reaction.” The rocket expels gases downward at high speed (action), and in response, the rocket is propelled upward (reaction). This principle enables space rockets to move even in the vacuum of space where no air resistance exists.

• Newton’s First Law: Deals with inertia, not propulsion.
• Newton’s Second Law: Explains force and acceleration but doesn’t directly describe rocket thrust.