UPSC NDA-1

41. Which one of the following metals is used in the filaments of photo-el

Which one of the following metals is used in the filaments of photo-electric cells that convert light energy into electric energy ?

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Tungsten
Copper
Rubidium
Aluminium
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Photo-electric cells, which convert light energy into electrical energy via the photoelectric effect, utilize materials with low work functions. Alkali metals are known for their low work functions, meaning they easily emit electrons when illuminated by light of sufficient frequency. Among the given options, Rubidium is an alkali metal and is used in photoemissive surfaces in some types of photocells or photomultiplier tubes. Caesium is also very commonly used, often alloyed with other elements.
Alkali metals have low work functions, making them suitable for use as photoemissive materials in devices that rely on the photoelectric effect.
Tungsten is a refractory metal with a high melting point and is used as a filament in incandescent bulbs, but it has a high work function. Copper and Aluminium are conductors but are not typically used as primary photoemissive surfaces for converting light to electricity in this manner. Photocathodes in modern photomultiplier tubes often use complex alloys or semiconductor materials for optimized sensitivity to different wavelengths of light.

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42. Which one of the following metals is alloyed with sodium to transfer h

Which one of the following metals is alloyed with sodium to transfer heat in a nuclear reactor ?

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Potassium
Calcium
Magnesium
Strontium
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Liquid metals, particularly alloys of Sodium and Potassium (NaK), are used as coolants in certain types of nuclear reactors, especially fast breeder reactors, because they have high thermal conductivity and remain liquid over a wide temperature range. The NaK alloy is used to transfer heat from the reactor core. Therefore, Potassium is the metal alloyed with sodium for this purpose among the given options.
Sodium-Potassium (NaK) alloy is a common liquid metal coolant in nuclear reactors due to its excellent heat transfer properties.
NaK alloys are eutectic, meaning they have a lower melting point than either sodium or potassium individually, making them liquid at room temperature (for certain compositions). This is advantageous for starting up and shutting down the reactor.

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43. Which one of the following alkali metals has lowest melting point ?

Which one of the following alkali metals has lowest melting point ?

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Sodium
Potassium
Rubidium
Caesium
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The alkali metals are elements in Group 1 of the periodic table: Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Caesium (Cs), and Francium (Fr). As we move down the group, the atomic size increases. The metallic bond strength weakens with increasing atomic size because the valence electron is further from the nucleus and shielded by more inner electron shells, leading to lower melting and boiling points. Therefore, among the given options (Sodium, Potassium, Rubidium, Caesium), Caesium (Cs) has the largest atomic size and thus the weakest metallic bonding, resulting in the lowest melting point.
Melting points of alkali metals decrease down the group due to decreasing metallic bond strength as atomic size increases.
The melting points of the given alkali metals are approximately: Sodium (97.8 °C), Potassium (63.5 °C), Rubidium (39.3 °C), and Caesium (28.5 °C). Caesium’s melting point is just above room temperature.

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44. Two convex lenses with power 2 dioptre are kept in contact with each o

Two convex lenses with power 2 dioptre are kept in contact with each other. The focal length of the combined lens system is

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0.10 m
2 m
4 m
0.25 m
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When two thin lenses are kept in contact, the total power of the combination is the sum of the individual powers. The power of a lens (P) is given by the reciprocal of its focal length (f) in meters, i.e., P = 1/f.
Given the power of each lens is 2 Dioptre, the total power of the combined system is $P_{total} = P_1 + P_2 = 2 \, \text{D} + 2 \, \text{D} = 4 \, \text{D}$.
The focal length of the combined lens system (F) is the reciprocal of the total power: $F = 1 / P_{total} = 1 / 4 \, \text{D} = 0.25 \, \text{meters}$.
The power of lenses is additive when they are placed in contact. This property is useful for designing lens systems.
Dioptre (D) is the unit of power of a lens, defined as the reciprocal of the focal length in meters. A convex lens has positive power and focal length, while a concave lens has negative power and focal length.

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45. Which one of the following statements about magnetic field lines is NO

Which one of the following statements about magnetic field lines is NOT correct?

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They can emanate from a point
They do not cross each other
Field lines between two poles cannot be precisely straight lines at the ends
There are no field lines within a bar magnet
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Magnetic field lines form continuous closed loops. Outside a magnet, they are depicted as originating from the North pole and terminating at the South pole. Inside the magnet, the field lines continue from the South pole to the North pole, completing the loop. Therefore, there *are* magnetic field lines within a bar magnet.
Magnetic field lines are continuous and form closed loops, unlike electric field lines which start on positive charges and end on negative charges. This reflects the absence of magnetic monopoles.
Statement A is broadly correct in that lines emanate from the pole regions (which are not mathematical points but areas) of a magnet. Statement B is a fundamental property: field lines never cross each other, as this would imply two different directions for the magnetic field at a single point, which is impossible. Statement C is correct: field lines are generally curved, especially near the poles and in the region between them, only becoming approximately straight far from the magnet or in very specific configurations.

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46. Which one of the following statements about the refractive index of a

Which one of the following statements about the refractive index of a material medium with respect to air is correct ?

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It can be either positive or negative
It can have zero value
It is unity for all materials
It is always greater than one
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The refractive index of a material medium ($n_m$) with respect to air ($n_{air}$) is defined as the ratio of the speed of light in air ($v_{air}$) to the speed of light in the medium ($v_m$). Since the speed of light in any material medium is always less than the speed of light in vacuum (and approximately less than the speed of light in air), the ratio $v_{air} / v_m$ will always be greater than 1 for any material medium (except vacuum or air itself, where the refractive index is 1).
Refractive index is a measure of how much the speed of light is reduced in a medium compared to air/vacuum. A higher refractive index indicates a slower speed of light in the medium and greater bending of light.
Refractive index values are typically positive. A refractive index of zero or negative would imply unusual optical properties, not characteristic of common materials. The refractive index of air with respect to vacuum is slightly greater than 1, but often approximated as 1. The refractive index of a medium with respect to vacuum is always greater than or equal to 1.

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47. Which one of the following is an example of the force of gravity of th

Which one of the following is an example of the force of gravity of the earth acting on a vibrating pendulum bob ?

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Applied force
Frictional force
Restoring force
Virtual force
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When a pendulum bob is displaced from its equilibrium position, the force of gravity acting vertically downwards, combined with the tension in the string, results in a net force that pulls the bob back towards the equilibrium position. This force, which is always directed towards the equilibrium position and proportional to the displacement (for small angles), is known as the restoring force.
In simple harmonic motion, the restoring force is crucial as it is responsible for bringing the oscillating system back to its equilibrium state. For a simple pendulum, the component of gravity tangential to the arc of motion provides this restoring force.
Applied force is a general term for any external force. Frictional force opposes motion. Virtual force (or fictitious force) appears in non-inertial reference frames. Gravity itself is the fundamental force involved, but its component acting along the path of motion towards equilibrium is the restoring force in the context of oscillation.

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48. Which of the following statements about specific heat of a body is/are

Which of the following statements about specific heat of a body is/are correct ?

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  • 1. It depends upon mass and shape of the body
  • 2. It is independent of mass and shape of the body
  • 3. It depends only upon the temperature of the body

Select the correct answer using the code given below :

1 only
2 and 3
1 and 3
2 only
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The correct answer is (D) 2 only. Specific heat capacity is an intrinsic property of the material itself. It is defined as the amount of heat required to raise the temperature of a unit mass of a substance by one degree Celsius (or Kelvin). Therefore, it is independent of the mass and the shape of the *specific body* made from that substance. While specific heat capacity can vary slightly with temperature and pressure for a given substance, statement 3 claiming it depends *only* upon the temperature is inaccurate, as the fundamental material composition is the primary determinant. Statement 1 is definitively incorrect. Statement 2 correctly states that it is independent of mass and shape of the body.
Specific heat capacity is an intensive property of a substance, meaning it does not depend on the amount (mass) or form (shape) of the substance.
Heat capacity (C), in contrast to specific heat capacity (c), *does* depend on the mass of the body (C = mc). Specific heat capacity is a fundamental property used to compare how different substances store thermal energy. For example, water has a high specific heat capacity compared to metals.

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49. Chemical weathering of rocks is much greater in a place with

Chemical weathering of rocks is much greater in a place with

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cold and dry climate
hot and humid climate
hot and dry climate
cold and humid climate
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The correct answer is (B) hot and humid climate. Chemical weathering involves chemical reactions like hydrolysis, oxidation, carbonation, etc., which break down rocks. These reactions are significantly accelerated by higher temperatures and the presence of water. A hot climate increases the rate of chemical reactions, and a humid climate provides the necessary moisture (water) for these reactions to occur.
Chemical weathering is most effective in environments with high temperatures and abundant moisture, as these conditions promote the chemical breakdown of minerals in rocks.
In cold climates, physical weathering (like freeze-thaw) is more dominant. In dry climates, both chemical and physical weathering rates are generally lower due to lack of water, although some forms of weathering like exfoliation (due to thermal expansion/contraction) can occur. Hot and dry climates might see relatively faster chemical weathering than cold climates if some moisture is available, but hot and *humid* is the most conducive environment for chemical weathering.

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50. Which of the following is/are coastal erosional feature(s) ? 1. Notc

Which of the following is/are coastal erosional feature(s) ?

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  • 1. Notch
  • 2. Sea Arch
  • 3. Cliff
  • 4. Hook

Select the correct answer using the code given below :

1, 2 and 3
2, 3 and 4
2 and 3 only
1 only
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UPSC NDA-1 – 2018
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The correct answer is (A) 1, 2 and 3.
1. Notch: An undercut base of a cliff formed by wave erosion is an erosional feature.
2. Sea Arch: An archway formed in a headland by wave erosion is an erosional feature.
3. Cliff: A steep rock face often formed by the erosive action of waves on a coastline is an erosional feature.
4. Hook: A hook is a type of spit where the end is curved inwards due to wave refraction or opposing currents. A spit is a depositional feature made of accumulated sediment. Thus, a hook is fundamentally depositional.
Therefore, 1, 2, and 3 are coastal erosional features.
Coastal erosional features are shaped by the destructive forces of waves, currents, and tides on the coastline, while depositional features are built up by the accumulation of sediment.
Processes like hydraulic action, abrasion, attrition, and solution contribute to coastal erosion, leading to the formation of landforms like notches, caves, arches, stacks, stumps, cliffs, and wave-cut platforms. Depositional processes create landforms like beaches, spits, bars, tombolos, and dunes.

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