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Q1. Power of a Convex Lens
A convex lens is a transparent curved lens that is thicker at the center and thinner at the edges. This type of lens is also known as a converging lens because it bends light rays inward and brings them together at one point called the principal focus. Convex lenses are commonly used in spectacles, cameras, microscopes, telescopes, and magnifying glasses.
In this question, the focal length of the convex lens is given as 0.5 meter. The power of a lens tells us how strongly the lens can bend light rays. The SI unit of power is Dioptre (D).
The formula used to calculate the power of a lens is:
Power (P) = 1 / Focal Length (f)
Here,
f = 0.5 m
So,
P = 1 / 0.5
P = 2 D
Since the lens is convex, the power will always be positive. Therefore, the correct answer is +2 D.
The concept of lens power is extremely important in optics. A lens having smaller focal length bends light more strongly and therefore has greater power. A lens with larger focal length bends light less and has lower power.
Convex lenses are used in many daily life applications. People suffering from hypermetropia or long-sightedness use convex lenses in spectacles to see nearby objects clearly. Magnifying glasses also use convex lenses because they enlarge the image of small objects.
Camera lenses use combinations of convex and concave lenses to focus images properly. In microscopes and telescopes, convex lenses help scientists observe tiny objects and distant stars.
Students should remember:
- Convex lens → Positive power
- Concave lens → Negative power
The relationship between focal length and power is inverse. If focal length decreases, power increases. If focal length increases, power decreases.
This topic also helps students understand image formation, magnification, and optical instruments. It forms the base for advanced physics topics related to light and lenses.
Q2. Saturated Hydrocarbon
Hydrocarbons are compounds made only of carbon and hydrogen atoms. They are mainly divided into two categories:
- Saturated hydrocarbons
- Unsaturated hydrocarbons
A saturated hydrocarbon contains only single covalent bonds between carbon atoms. These compounds are also known as alkanes. In this question, the correct answer is Ethane.
Ethane has the chemical formula:
C₂H₆
In ethane, all carbon atoms are connected by single bonds. Therefore, it is called a saturated hydrocarbon.
Saturated hydrocarbons are generally stable and less reactive. They usually burn with a clean flame and are widely used as fuels.
Other options in the question are unsaturated hydrocarbons:
- Ethene contains a double bond.
- Ethyne contains a triple bond.
- Benzene contains alternating double bonds in a ring structure.
Unsaturated hydrocarbons are more reactive because of the presence of double or triple bonds.
Examples of saturated hydrocarbons include:
- Methane
- Ethane
- Propane
- Butane
These compounds are important components of natural gas and petroleum products.
Hydrocarbons are extremely useful in everyday life. They are used in:
- Cooking gas
- Vehicle fuel
- Industrial chemicals
- Plastic production
- Fertilizers
Students should remember:
- Single bond → Saturated hydrocarbon
- Double or triple bond → Unsaturated hydrocarbon
The study of hydrocarbons forms the foundation of organic chemistry. Carbon compounds are very important because they are found in fuels, medicines, plastics, food substances, and living organisms.
Ethane is a colorless gas and is often present in natural gas. It is also used in industries to produce chemicals such as ethene.
This topic helps students understand the classification of carbon compounds and the basic structure of organic molecules.
Q3. Electromagnetic Induction
Electromagnetic induction is one of the most important phenomena in physics. It is the process of producing electric current in a conductor by changing the magnetic field around it.
The correct answer is:
Producing induced current in a coil
This phenomenon was discovered by the scientist Michael Faraday. According to Faraday’s law, whenever the magnetic field linked with a conductor changes, an electric current is induced in the conductor.
The current produced is called induced current.
Electromagnetic induction can be observed when:
- A magnet is moved near a coil
- A coil is moved inside a magnetic field
- The magnetic field around a coil changes
In school experiments, a galvanometer connected to a coil shows deflection when a magnet is moved toward or away from the coil. This proves that electric current is generated.
Electromagnetic induction is the basic principle behind many electrical devices such as:
- Electric generators
- Transformers
- Bicycle dynamos
- Induction cookers
- Microphones
Electric generators in power stations use electromagnetic induction to produce electricity. Mechanical energy from water, wind, or steam turbines rotates coils inside magnetic fields and electricity is generated.
This discovery changed the modern world because almost all electricity generation systems are based on electromagnetic induction.
Students often confuse this concept with the magnetic effect of electric current.
Difference:
- Magnetic effect of current → Electric current produces magnetic field.
- Electromagnetic induction → Changing magnetic field produces electric current.
Electromagnetic induction is very important in modern technology. Without it, there would be no large-scale electricity production.
This topic also helps students understand renewable energy systems like hydroelectric plants and wind turbines.
Faraday’s contribution to science is considered one of the greatest achievements in physics because it led to the development of electrical engineering and modern power systems.
Q4. Reflex Action
A reflex action is a quick, automatic, and involuntary response of the body to a stimulus. Reflex actions help protect the body from sudden harm and danger.
In this question, the correct answer is:
Blinking of eyes
Blinking occurs automatically to protect the eyes from dust, bright light, insects, or harmful particles. It happens without conscious thinking.
Reflex actions are controlled mainly by the spinal cord through a pathway known as the reflex arc.
A reflex arc consists of:
- Receptor
- Sensory neuron
- Spinal cord
- Motor neuron
- Effector organ
The pathway works very quickly so that the body can respond immediately.
Examples of reflex actions:
- Pulling hand away from fire
- Sneezing
- Coughing
- Knee jerk reaction
- Blinking of eyes
These actions occur automatically and help prevent injuries.
The other options in the question such as thinking, writing, and dancing require conscious control by the brain. Therefore, they are not reflex actions.
Reflex actions are important because they save time and protect the body. If a person touches a hot object, the hand immediately moves away before the brain fully processes the pain.
This rapid response system is very useful for survival.
Doctors often test reflex actions to check the health of the nervous system. Reflex tests help identify nerve damage or neurological problems.
Students should remember:
- Reflex action = Fast automatic response
- Controlled mainly by spinal cord
- Helps in protection
The nervous system is one of the most complex systems in the human body. Reflex actions show how efficiently the body responds to environmental changes.
This topic is very important in biology because it explains coordination and control in living organisms.
Q5. Layers of the Atmosphere
The atmosphere is the blanket of gases surrounding the Earth. It protects life on Earth and plays a very important role in maintaining environmental balance.
The atmosphere contains gases such as:
- Nitrogen
- Oxygen
- Carbon dioxide
- Argon
- Water vapor
The atmosphere is divided into five major layers. Therefore, the correct answer is:
5 layers
The five layers are:
- Troposphere
- Stratosphere
- Mesosphere
- Thermosphere
- Exosphere
Troposphere
This is the lowest layer of the atmosphere and the layer in which humans live. All weather changes such as rain, wind, clouds, and storms occur here.
Important features:
- Contains most of the air
- Contains water vapor
- Supports life
- Temperature decreases with height
Aeroplanes generally avoid the lower part of this layer during bad weather conditions.
Stratosphere
The stratosphere lies above the troposphere. It contains the ozone layer, which protects Earth from harmful ultraviolet rays of the Sun.
Important features:
- Stable air conditions
- Commercial aircraft often fly here
- Contains ozone layer
Without the ozone layer, harmful solar radiation could damage living organisms.
Mesosphere
The mesosphere is the coldest atmospheric layer.
Important features:
- Meteors burn in this layer
- Very low temperature
- Thin air
This layer protects Earth because most meteors get destroyed before reaching the surface.
Thermosphere
The thermosphere becomes extremely hot because it absorbs high-energy solar radiation.
Important features:
- Very high temperature
- Satellites move here
- Space stations orbit in this region
The ionosphere, which helps in radio communication, is present in this layer.
Exosphere
This is the outermost layer of the atmosphere. It gradually merges into outer space.
Important features:
- Extremely thin air
- Light gases present
- Transition between Earth and space
The atmosphere is very important because it:
- Provides oxygen for breathing
- Maintains Earth’s temperature
- Protects against harmful radiation
- Supports the water cycle
- Helps in weather formation
Without the atmosphere, life on Earth would not be possible.
Students should remember the sequence of layers from Earth upward:
Troposphere → Stratosphere → Mesosphere → Thermosphere → Exosphere
A simple memory trick is:
The
Sky
Makes
The
Earth beautiful
The atmosphere also plays a major role in climate and environmental protection. Human activities such as pollution and greenhouse gas emissions are affecting the atmosphere and causing global warming.
Understanding atmospheric layers helps students learn about:
- Weather
- Climate
- Satellites
- Space science
- Environmental conservation
This topic is very useful in geography, environmental science, and physics.
Conclusion
These science questions cover important topics from physics, chemistry, biology, and environmental science. Students learn about lenses, hydrocarbons, electricity, reflex actions, and atmospheric layers through these concepts.