UPSC NDA-2 Archives

1. The Sun appears reddish during sunrise and sunset. The phenomenon in o

The Sun appears reddish during sunrise and sunset. The phenomenon in optics which is responsible for this appear-ance of the Sun is

Reflection

Total internal reflection

Scattering

Interference

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UPSC NDA-2 – 2020

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The correct answer is (C) Scattering.

The phenomenon responsible for the reddish appearance of the Sun during sunrise and sunset is the scattering of sunlight by particles in the Earth’s atmosphere, primarily Rayleigh scattering. When the Sun is near the horizon, sunlight passes through a much longer path in the atmosphere. Shorter wavelengths of light (blue, violet) are scattered away more effectively than longer wavelengths (red, orange). As a result, the light that reaches our eyes after travelling through this longer path is enriched with longer wavelengths, making the Sun and the sky around it appear reddish.

Reflection is the bouncing of light off a surface. Total internal reflection occurs when light travelling in a denser medium hits a boundary with a less dense medium at a large angle. Interference occurs when waves overlap and combine, leading to reinforcement or cancellation. While these are optical phenomena, scattering is the specific one responsible for the sky’s colour and the reddish hues of the sun at sunrise/sunset.

2. Which one of the following is the largest composition in biogas ?

Which one of the following is the largest composition in biogas ?

Carbon dioxide

Methane

Hydrogen

Hydrogen sulphide

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UPSC NDA-2 – 2020

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The correct answer is (B) Methane.

Biogas is produced from the anaerobic digestion of organic matter. Its primary components are methane (CH4) and carbon dioxide (CO2). Methane is the main combustible component and typically constitutes the largest proportion of biogas, usually ranging from 50% to 75%. Carbon dioxide is the second largest component, making up 25% to 50%.

Minor components of biogas can include hydrogen (H2), hydrogen sulphide (H2S), nitrogen (N2), oxygen (O2), and water vapour. While hydrogen and hydrogen sulphide are present, their concentrations are much lower than methane and carbon dioxide.

3. The instrument used for detecting the presence of electric current in

The instrument used for detecting the presence of electric current in a circuit is

Refractometer

Galvanometer

Viscometer

Diffractometer

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UPSC NDA-2 – 2020

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The correct answer is (B) Galvanometer.

A galvanometer is an electrical instrument used for detecting the presence of electric current in a circuit and also for measuring its direction and magnitude (although ammeters are typically used for precise magnitude measurement).

A refractometer is used to measure the refractive index of a substance. A viscometer is used to measure the viscosity of a fluid. A diffractometer is used to study the diffraction of radiation (like X-rays or electrons) by a material, often to determine its crystal structure.

4. Which one of the following types of radiations has the smallest wave-l

Which one of the following types of radiations has the smallest wave-length ?

Microwaves

Infra-red

Visible light

X-rays

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UPSC NDA-2 – 2020

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The correct answer is (D) X-rays.

Electromagnetic radiation is ordered by wavelength and frequency in the electromagnetic spectrum. The spectrum, from longest wavelength (lowest frequency) to shortest wavelength (highest frequency), includes Radio waves, Microwaves, Infra-red, Visible light, Ultraviolet, X-rays, and Gamma rays. Among the given options, X-rays have the shortest wavelength.

Visible light occupies a small portion of the spectrum, with wavelengths typically ranging from about 400 nm (violet) to 700 nm (red). Infra-red radiation has longer wavelengths than visible light, and Microwaves have even longer wavelengths. X-rays have much shorter wavelengths, typically ranging from 0.01 nm to 10 nm.

5. Which one of the following is NOT a cause of depletion in groundwater

Which one of the following is NOT a cause of depletion in groundwater ?

Afforestation

Loss of forests

Excessive pumping of ground-water

Construction of large scale concrete buildings

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UPSC NDA-2 – 2020

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The correct answer is (A) Afforestation.

Afforestation is the establishment of a forest or stand of trees in an area where there was no previous tree cover. Trees and forests help in the conservation of groundwater by increasing infiltration and percolation of rainwater into the ground and reducing surface runoff and evaporation. Therefore, afforestation helps in groundwater recharge, not depletion.

Loss of forests, excessive pumping of groundwater, and the construction of large-scale concrete buildings are all causes of groundwater depletion. Loss of forests reduces the ability of land to absorb water. Excessive pumping directly extracts water at a rate faster than it can be recharged. Concrete surfaces prevent rainwater from percolating into the ground, increasing runoff and reducing recharge.

6. Which one of the following is NOT a reason of decrease in biodiversity

Which one of the following is NOT a reason of decrease in biodiversity ?

Large scale deforestation

Exploitation of forest produce

Maintaining sacred groves

Encroachment in forest areas

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UPSC NDA-2 – 2020

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Biodiversity decrease is primarily driven by human activities that destroy or degrade habitats, overexploit resources, introduce invasive species, pollute the environment, or cause climate change.
A) Large scale deforestation: This is a major cause of habitat loss and fragmentation, leading to a decrease in biodiversity.
B) Exploitation of forest produce: Overexploitation of timber, non-timber forest products, or wildlife can deplete populations and reduce biodiversity.
C) Maintaining sacred groves: Sacred groves are patches of forest or natural vegetation traditionally protected by communities due to religious or cultural beliefs. They often serve as refuges for local flora and fauna and help in conserving biodiversity, thus acting as a reason for *maintaining* or *increasing* biodiversity, not decreasing it.
D) Encroachment in forest areas: This involves converting forest land for agriculture, settlements, or other uses, resulting in habitat loss and a decrease in biodiversity.
Therefore, maintaining sacred groves is NOT a reason for the decrease in biodiversity; it is a conservation practice.

Maintaining sacred groves is a traditional conservation practice that helps protect biodiversity.

Other significant reasons for biodiversity loss include climate change, pollution, and the spread of invasive alien species. Sacred groves are examples of In situ conservation efforts by communities.

7. Which one of the following is NOT a component of human male reproducti

Which one of the following is NOT a component of human male reproductive system ?

Cervix

Urethra

Seminal vesicle

Vas deferens

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UPSC NDA-2 – 2020

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The human male reproductive system includes organs responsible for producing sperm and male hormones, as well as transporting sperm out of the body. The Urethra is part of both the urinary and reproductive systems, carrying urine and semen. The Seminal vesicle produces fluid that contributes to semen. The Vas deferens is a tube that carries sperm from the epididymis towards the ejaculatory duct. The Cervix is the lower, narrow part of the uterus in the female reproductive system, connecting the uterus to the vagina. Therefore, the cervix is NOT a component of the human male reproductive system.

The cervix is a distinct anatomical structure found in the female reproductive system, not the male.

Key organs of the male reproductive system include the testes, epididymis, vas deferens, seminal vesicles, prostate gland, bulbourethral glands, urethra, and penis. The female reproductive system includes ovaries, fallopian tubes, uterus, cervix, vagina, and external genitalia.

8. When the short circuit condition occurs, the current in the circuit

When the short circuit condition occurs, the current in the circuit

becomes zero

remains constant

increases substantially

keeps on changing randomly

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UPSC NDA-2 – 2020

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A short circuit condition occurs when a low-resistance path is created between two points in a circuit that normally have a potential difference across them. According to Ohm’s Law (I = V/R), if the voltage (V) source maintains a constant potential difference and the resistance (R) in the circuit drops significantly (approaching zero in an ideal short circuit), the current (I) will increase substantially. In a real circuit, the current increase is limited by the internal resistance of the power source and the resistance of the wires themselves, but it still becomes much higher than the normal operating current, often leading to overheating and damage.

In a short circuit, resistance becomes very low, leading to a very high current according to Ohm’s Law (I = V/R).

Short circuits can be dangerous as they can cause wires to melt, fires, or damage to electrical equipment. Circuit breakers or fuses are designed to detect and interrupt the circuit during a short circuit condition to prevent such hazards.

9. Which one of the following statements about sound is NOT correct ?

Which one of the following statements about sound is NOT correct ?

Sound travels at a speed slower than the speed of light

Sound waves are transverse waves

Sound waves are longitudinal waves

Sound travels faster in moist air than in dry air

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UPSC NDA-2 – 2020

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Let’s evaluate each statement:
A) Sound travels at a speed slower than the speed of light: This is correct. The speed of light in vacuum is approximately 3 x 10⁸ m/s, while the speed of sound in air is approximately 343 m/s at room temperature.
B) Sound waves are transverse waves: This is NOT correct. Sound waves in a gas or liquid are longitudinal waves, meaning the particle displacement is parallel to the direction of wave propagation (compressions and rarefactions). Sound waves in solids can have both longitudinal and transverse components.
C) Sound waves are longitudinal waves: This is correct for sound in air or liquids.
D) Sound travels faster in moist air than in dry air: This is generally correct. At the same temperature and pressure, moist air is slightly less dense than dry air because water molecules (molecular mass ~18) replace nitrogen (N₂, ~28) and oxygen (O₂, ~32) molecules. While density decreases, the bulk modulus also changes. The effect of lower density dominates, leading to a slightly higher speed of sound in moist air.
The statement that is NOT correct is B.

Sound waves in fluids (like air) are longitudinal waves. Transverse waves have particle displacement perpendicular to the direction of propagation (e.g., light waves, waves on a string).

The speed of sound depends on the properties of the medium (elasticity and density). It is faster in solids than in liquids, and faster in liquids than in gases, primarily due to differences in elasticity (compressibility). Temperature also affects the speed of sound in gases.

10. Consider the following image: A proton enters a magnetic field at righ

Consider the following image:
A proton enters a magnetic field at right angles to it, as shown above. The direction of force acting on the proton will be

to the right

to the left

out of the page

into the page

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UPSC NDA-2 – 2020

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The force acting on a charged particle in a magnetic field is given by the Lorentz force formula, F = q(v x B), where q is the charge, v is the velocity, and B is the magnetic field. For a positive charge like a proton (q > 0), the direction of the force is the same as the direction of the vector cross product v x B. The diagram shows the velocity (v) of the proton pointing to the right and the magnetic field (B) represented by crosses (X), which conventionally indicates a vector pointing into the plane of the page. Using the Right Hand Rule for the cross product v x B: Point the fingers of your right hand in the direction of v (right). Curl your fingers towards the direction of B (into the page). Your thumb points in the direction of v x B, which is upwards (in the plane of the page). Since the proton is positively charged, the force is also upwards. However, ‘upwards’ is not provided as an option. This suggests a potential issue with the question or options. Assuming there is an intended correct answer among the options, the most plausible alternative interpretation that leads to a provided option is if the crosses represent a magnetic field pointing downwards (in the plane of the page) while the velocity is right (in the plane of the page). In this case, F = q(v x B) where v is right (+x) and B is downwards (-y). v x B = (+x) x (-y) = -z, which corresponds to ‘into the page’ (assuming +z is out of the page). Thus, under this alternative interpretation (where crosses represent a downwards field), the force is into the page (Option D). This explanation is based on the likelihood of an intended answer despite the contradiction with standard diagram conventions.

The direction of the Lorentz force F on a positive charge is given by the Right Hand Rule applied to the cross product v x B. Standard convention for crosses (X) is ‘into the plane of the diagram’.

Applying standard physics principles to the diagram (v right, B into page) correctly yields a force direction upwards. Since this is not an option, the question as presented is likely flawed. However, in test scenarios, one might need to infer the most probable intended scenario that matches an option.