Refraction of light is the phenomenon where light changes its direction when it passes from one transparent medium to another, resulting in a change in its speed. This bending of light occurs because light travels at different speeds in different media such as air, water, or glass.

When a ray of light travels from one medium to another at an angle other than 90°, the change in speed causes the ray to change direction at the boundary between the two media. This change in direction is called refraction.

For example, when a pencil is placed partly in water, it appears bent or broken at the water surface due to refraction. This is because light rays coming from the pencil bend when they pass from water to air, altering the apparent position of the pencil.

Key Points About Refraction:

• Refraction occurs when light passes from one medium to another with a different optical density.
• It causes the light ray to bend towards or away from the normal (an imaginary line perpendicular to the surface) depending on whether the light is entering a denser or rarer medium.
• The amount of bending depends on the refractive indices of the two media.
• Refraction is responsible for phenomena such as the apparent bending of objects in water, formation of rainbows, and focusing of light by lenses.

Laws of Refraction:

1. The incident ray, refracted ray, and the normal to the interface of two media lie in the same plane.
2. The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for a given pair of media, known as the refractive index.

Mathematically,

Where i is the angle of incidence, r is the angle of refraction, and μ is the refractive index.

Importance of Refraction:
Refraction helps in the functioning of lenses used in glasses, cameras, microscopes, and telescopes. It also explains many natural phenomena like the formation of rainbows and mirages.

In summary, refraction of light is a fundamental concept that explains how light bends when traveling between different transparent materials, influencing how we see the world and enabling many optical devices.

What Are Fossil Fuels?

Fossil fuels are energy sources formed from the remains of ancient plants and animals that lived millions of years ago. Over time, these remains got buried under layers of mud, rocks, and sediments. Due to heat and pressure over millions of years, these organic materials changed into coal, oil, and natural gas. Because this process takes such a very long time, fossil fuels are considered non-renewable.

Why Are Fossil Fuels Called Exhaustible?

Fossil fuels are called exhaustible or non-renewable natural resources due to the following reasons:

1. Limited Formation Time: The process of forming fossil fuels takes millions of years, which is far longer than a human lifetime. So, once we use them up, they cannot be replaced quickly.
2. Finite Quantity: The amount of fossil fuels available on Earth is limited. We have a fixed reserve, and continued usage depletes this reserve permanently.
3. Irreplaceable Once Used: When fossil fuels are burned for energy, the stored carbon combines with oxygen to form carbon dioxide, releasing energy but destroying the fossil fuel itself. This process is irreversible.
4. Increasing Demand: With growing population and industrial activities, the demand for fossil fuels keeps increasing. This speeds up their exhaustion.

Importance of Fossil Fuels and Environmental Impact

Fossil fuels are important because they provide a high amount of energy and are currently the cheapest and most accessible energy sources for many countries. However, burning fossil fuels releases greenhouse gases like carbon dioxide, which contribute to global warming and climate change. This environmental impact is another reason why it’s important to reduce our dependence on them.

How Can We Conserve Fossil Fuels?

Since fossil fuels are exhaustible, conserving them is necessary to ensure availability for future generations. Here are some ways to do this:

• Efficient Energy Use: Using energy-efficient appliances and vehicles reduces fossil fuel consumption.
• Using Public Transport: Reduces gasoline and diesel use.
• Switching to Renewable Energy: Solar, wind, and hydro power are renewable and eco-friendly alternatives.
• Reducing Waste: Simple actions like turning off lights when not needed save energy.
• Promoting Awareness: Educating people about fossil fuels’ limits helps encourage conservation.

Conclusion

Fossil fuels are exhaustible natural resources because they take millions of years to form and once consumed, cannot be replaced. Due to their limited quantity and environmental harm, it is important for students and everyone to understand the need for conservation and the shift towards renewable energy. By using fossil fuels wisely and adopting green energy sources, we can build a sustainable future.

NCERT Science- Related Question and Answer

Q. Explain why fossil fuels are exhaustible natural resources.

Fossil fuels are called exhaustible natural resources because they cannot be replaced once they are used up. They were formed millions of years ago from the remains of plants and animals buried deep under the earth. Since it takes so much time for fossil fuels like coal, oil, and natural gas to form, we cannot make more of them quickly. When we burn fossil fuels for energy, they get used up and once finished, they will be gone forever. That is why we say fossil fuels are exhaustible and limited in quantity. We need to use them carefully and also think about using other renewable sources of energy to save them for the future.

Q. How are fossil fuels formed?

Fossil fuels are formed from the remains of dead plants and animals that lived millions of years ago. When these organisms died, they settled in water bodies like swamps and oceans, where they were buried under layers of mud and sediments. Because of the low oxygen conditions, their remains did not completely decompose. Over millions of years, heat and pressure from the layers of sediments caused these remains to change chemically and physically.

This process turned plant remains into coal and animal/plankton remains into oil and natural gas. The entire process takes millions of years, which is why fossil fuels are called non-renewable resources.

Fossil Fuels can be made in the laboratory. True or False?

False. Fossil fuels cannot be created in the laboratory because their formation requires millions of years of natural processes involving heat, pressure, and the decomposition of ancient organic matter, which cannot be replicated quickly in a lab.

What is a Balanced Chemical Equation?

A balanced chemical equation represents a chemical reaction with equal numbers of each type of atom on both sides: the reactants (what you start with) and the products (what you get) of the reaction.
This means that atoms are not created or destroyed; they are simply rearranged.

Example:

Unbalanced equation:

H₂ + O₂ → H₂O

Here, you have:

• 2 hydrogen atoms on both sides (H₂)
• 2 oxygen atoms on the left (O₂), but only 1 on the right (H₂O)

This equation does not follow the rules because the number of oxygen atoms is different on both sides.

Balanced equation:

2H₂ + O₂ → 2H₂O

Here:

• There are 4 hydrogen atoms and 2 oxygen atoms on both sides.

Why Is Balancing Chemical Equations Important?

Balancing chemical equations follows the law of conservation of mass. This law states that matter cannot be created or destroyed in a chemical reaction. Every atom present at the start must be present at the end, just rearranged.

In simple words:
Balancing makes sure all the “ingredients” and “results” match up perfectly.

How to Balance a Chemical Equation: Step-by-Step

1. Write the skeleton equation.
   Example: H₂ + O₂ → H₂O
2. Count the number of atoms of each element on both sides.
   Left: H = 2, O = 2
   Right: H = 2, O = 1
3. Add coefficients (numbers) in front of molecules to balance them.
   Try: 2H₂ + O₂ → 2H₂O
4. Check both sides again.
   Left: H = 4, O = 2
   Right: H = 4, O = 2
   Both sides are balanced!
5. Make sure all coefficients are in the simplest whole number ratio.

More Examples of Balanced Chemical Equations

Tips

• Always balance metals, then non-metals, and balance hydrogen and oxygen last.
• Never change the subscripts (small numbers in chemical formulas), only the coefficients (numbers in front).
• Practice with lots of equations to get better and faster at balancing!
• Remember: A balanced chemical equation is the foundation of solving all types of chemistry problems.

Balanced chemical equations are the backbone of chemistry—they help you understand reactions, perform experiments correctly, and score better in exams. Whenever you write a chemical reaction, make sure the equation is balanced for both understanding and marks!

NCERT Class X Science Question and answer

What is a balanced chemical equation? Why should chemical equations be balanced?

Answer:
“A balanced chemical equation is a chemical reaction equation in which the number of atoms of each element is the same on both the reactant and product sides. It follows the law of conservation of mass, meaning no atoms are lost or gained, only rearranged.
Example: H₂ + O₂ → H₂O is balanced because it has 4 hydrogen atoms and 2 oxygen atoms on both sides.

Chemical equations should be balanced to satisfy the law of conservation of mass, ensuring that matter is neither created nor destroyed during a chemical reaction.”

Why do stars twinkle short answer

Stars twinkle because their light passes through the Earth’s atmosphere, which is made of layers of air with different temperatures and densities moving unpredictably. This causes the light to bend and change direction slightly multiple times before reaching our eyes, making the stars appear to flicker or twinkle. Stars near the horizon twinkle more because their light travels through more atmosphere. Planets don’t twinkle as much because they appear as larger disks, so the atmospheric effects average out, making their light steadier.

तारे टिमटिमाते नहीं हैं

तारे टिमटिमाते हुए दिखाई देते हैं क्योंकि हमारी धरती के चारों ओर की हवा हमेशा हिलती-डुलती रहती है। जब तारे की रोशनी हमें पहुँचती है, तो उसे इस हिलती-डुलती हवा से गुजरना पड़ता है। हवा की वजह से तारे की रोशनी थोड़ी कांपती या लड़खड़ाती है, इसलिए ऐसा लगता है जैसे तारे चमक रहे हों या झिलमिला रहे हों।

यह वैसा ही है जैसे आप पोखर में तैरते पानी के नीचे कुछ देखते हो और वह थोड़ा हिलता-डुलता हुआ दिखाई देता है।

तारे आसमान के किनारे ज्यादा टिमटिमाते हैं क्योंकि उस जगह से उनकी रोशनी को ज्यादा हवा से गुजरना होता है। ग्रह (प्लैनेट) कम टिमटिमाते हैं क्योंकि वे तारे से करीब होते हैं और उनकी रोशनी ज्यादा स्थिर होती है।

असल में तारे टिमटिमाते नहीं हैं, यह केवल हमारे आस-पास की हवा के कारण ऐसा लगता है।

Easy to understand: Why Stars Twinkle

Stars look like they are twinkling or blinking because the air around the Earth moves a lot. When the light from a star comes to us, it has to travel through the air, and this air moves like waves in the ocean. Because of this, the star’s light gets a little shaky and looks like it is flickering or shining on and off.

It’s a bit like when you look at something at the bottom of a swimming pool, and it looks wiggly because of the water moving. The same thing happens with the star’s light because of the air moving.

Stars twinkle more when they are near the edge of the sky because their light goes through more air. Planets don’t twinkle as much because they look bigger and their light doesn’t get shaky as much.

So stars don’t really blink, but it looks like they do because of the air moving around us!

Detail Answer on- Why do stars twinkle Class 10 NCERT

Stars appear to twinkle because of the Earth’s atmosphere. When the light from a star travels through our turbulent atmosphere, it passes through layers of air that are moving, with varying temperatures and densities. This causes the light to bend or refract multiple times in different directions before reaching our eyes. The result is that the star’s brightness and position seem to flicker or change rapidly, creating the twinkling effect, scientifically called “atmospheric scintillation” or “stellar scintillation.”

Stars twinkle more when they are near the horizon because their light passes through more layers of the atmosphere compared to when they are overhead. This twisting and bending of light waves cause the star’s light to flicker in brightness and sometimes even color.

Planets, on the other hand, do not twinkle as much because they are closer and appear as small disks rather than point sources of light. Their larger apparent size means the atmospheric distortions average out, making their light appear steadier.

In summary, stars do not actually twinkle themselves; it is the Earth’s atmosphere that causes the twinkling effect as star light is bent and distorted while traveling to our eyes.