UPSC CDS-2 Archives

1. Which one of the following statements about phloem is correct?

Which one of the following statements about phloem is correct?

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Phloem transports water and minerals.

Phloem transports photosynthetic products.

Phloem is a simple tissue.

Phloem gives support to the plant.

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Phloem is the vascular tissue in plants responsible for transporting sugars (primarily sucrose), produced during photosynthesis, from the leaves (source) to other parts of the plant (sink) where they are needed for growth or storage.

Xylem transports water and minerals from the roots upwards. Phloem transports organic nutrients, mainly sugars, throughout the plant. Phloem is a complex tissue made of sieve elements, companion cells, phloem parenchyma, and phloem fibres.

Simple tissues are composed of only one type of cell (e.g., parenchyma, collenchyma, sclerenchyma). Phloem, like xylem, is a complex tissue. Support in plants is primarily provided by sclerenchyma and collenchyma tissues.

2. Which one among the following is a non-conventional source of energy?

Which one among the following is a non-conventional source of energy?

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Petroleum

Coal

Radioactive elements

Solar energy

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Solar energy is considered a non-conventional or renewable source of energy.

Conventional energy sources are typically fossil fuels (coal, petroleum, natural gas) and nuclear energy, which are finite or use processes developed conventionally. Non-conventional sources are renewable and environmentally friendly, such as solar, wind, geothermal, tidal, and biomass energy.

Radioactive elements are used in nuclear power plants, which is often categorised as a conventional source due to its established technology and non-renewable fuel source (uranium).

3. Which one of the following combinations of source and screen would pro

Which one of the following combinations of source and screen would produce sharpest shadow of an opaque object?

A point source and an opaque screen

An extended source and an opaque screen

A point source and a transparent screen

An extended source and a transparent screen

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The sharpness of a shadow depends on the size and type of the light source and the properties of the screen. A point source of light produces a sharp shadow with a well-defined edge, called the umbra. An extended source of light produces a shadow with a dark central region (umbra) and a partially illuminated region around it (penumbra), resulting in a blurry edge. An opaque object is necessary to block the light and form a shadow. The screen needs to be opaque to display the shadow, as light would pass through a transparent screen. Therefore, the combination of a point source and an opaque screen produces the sharpest shadow of an opaque object.

A point source of light creates a sharp shadow (umbra) without a penumbra, unlike an extended source which creates both umbra and penumbra.

The size and distance of the object and the screen also affect the shadow’s size and shape, but the sharpness is primarily determined by the nature of the light source (point vs. extended) and the screen (opaque vs. transparent). An opaque screen is essential to intercept the light and make the shadow visible.

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4. Which one of the following phenomena verifies the fact that light trav

Which one of the following phenomena verifies the fact that light travels much faster than sound?

Twinkling of stars in night sky

Lighting of a matchstick

Thunderstorm

Mirage

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The phenomenon of a thunderstorm clearly demonstrates that light travels much faster than sound. During a thunderstorm, lightning is a visual flash of light, and thunder is the sound produced by the rapid expansion of air heated by the lightning discharge. We always see the lightning first and then hear the thunder after a noticeable delay. This delay is because light travels at approximately 3 x 10⁸ meters per second, while sound travels at approximately 343 meters per second in air (at 20°C), a speed difference of about a million times.

The time difference between seeing lightning and hearing thunder during a thunderstorm proves that light travels significantly faster than sound.

Twinkling of stars is due to atmospheric refraction and unrelated to the speed of sound. Lighting a matchstick involves processes happening over very small distances where the speed difference isn’t noticeable. A mirage is an optical phenomenon related to atmospheric refraction and temperature gradients, not the speed difference between light and sound.

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5. When a light ray enters into glass medium from water at an angle of in

When a light ray enters into glass medium from water at an angle of incidence 0°, what would be the angle of refraction?

90°

45°

0°

The ray will not enter at all

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When a light ray enters a medium from another medium at an angle of incidence of 0°, this means the ray is incident normally (perpendicularly) to the boundary surface between the two media. According to Snell’s Law of Refraction, n₁ sin(i) = n₂ sin(r), where n₁ and n₂ are the refractive indices of the two media, i is the angle of incidence, and r is the angle of refraction. If i = 0°, then sin(i) = sin(0°) = 0. The equation becomes n₁ * 0 = n₂ sin(r), which simplifies to 0 = n₂ sin(r). Since the refractive index of glass (n₂) is not zero, sin(r) must be zero. The angle whose sine is zero is 0°. Therefore, the angle of refraction (r) is 0°. This means the ray passes straight through the boundary without bending.

A light ray incident normally on a boundary between two media passes undeviated, meaning the angle of refraction is also 0°.

Refraction, or the bending of light, occurs when light changes speed as it passes from one medium to another at an angle. When the incidence is normal (angle of incidence = 0), the change in speed still occurs, but there is no change in direction.

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6. A luminous object is placed at a distance of 40 cm from a converging l

A luminous object is placed at a distance of 40 cm from a converging lens of focal length 25 cm. The image obtained in the screen is

erect and magnified

erect and smaller

inverted and magnified

inverted and smaller

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For a converging lens, the focal length is positive, f = +25 cm. The luminous object is placed at a distance u = -40 cm (object distance is negative by convention for real objects). We can use the lens formula 1/f = 1/v – 1/u to find the image distance (v).
1/25 = 1/v – 1/(-40)
1/25 = 1/v + 1/40
1/v = 1/25 – 1/40 = (8 – 5) / 200 = 3 / 200
v = 200/3 cm ≈ +66.7 cm.
Since v is positive, the image is real and formed on the opposite side of the lens from the object, which can be obtained on a screen. Real images formed by a single converging lens are always inverted.
The magnification is given by m = v/u = (200/3) / (-40) = -200 / 120 = -5/3 ≈ -1.67.
Since |m| = 5/3 > 1, the image is magnified. The negative sign indicates that the image is inverted. Thus, the image obtained on the screen is inverted and magnified.

For a converging lens, when the object is placed between f and 2f, a real, inverted, and magnified image is formed beyond 2f. In this case, u = 40 cm, which is between f (25 cm) and 2f (50 cm).

If the object were placed beyond 2f, the image would be real, inverted, and diminished. If the object were at 2f, the image would be real, inverted, and of the same size at 2f on the other side. If the object were between the optical center and f, the image would be virtual, erect, and magnified, formed on the same side as the object.

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7. Which of the following lenses will bend the light rays through largest

Which of the following lenses will bend the light rays through largest angle?

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Lens with power +2·0 D

Lens with power +2·5 D

Lens with power –1·5 D

Lens with power –2·0 D

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The power of a lens is a measure of its ability to converge or diverge light rays. It is defined as the reciprocal of the focal length in meters (P = 1/f). A lens with a higher absolute value of power bends light rays through a larger angle. Comparing the absolute values of the given powers: |+2.0 D| = 2.0 D, |+2.5 D| = 2.5 D, |-1.5 D| = 1.5 D, |-2.0 D| = 2.0 D. The largest absolute power is 2.5 D, corresponding to the lens with power +2.5 D. Therefore, this lens will bend the light rays through the largest angle.

The power of a lens (absolute value) indicates the extent to which it bends light; higher power means greater bending.

The sign of the power indicates the type of lens: positive power for a converging (convex) lens and negative power for a diverging (concave) lens. Converging lenses bring parallel rays together, while diverging lenses spread them out. However, the magnitude of bending is determined by the absolute value of the power.

8. Silver articles turn black when kept in the open for longer time due t

Silver articles turn black when kept in the open for longer time due to the formation of

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H₂S

AgS

AgSO₄

Ag₂S

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Silver articles tarnish and turn black when exposed to the air for a prolonged period. This tarnishing is primarily due to the reaction of silver (Ag) with hydrogen sulfide (H₂S) gas present in the atmosphere (often produced by the decomposition of organic matter or industrial pollution). This reaction forms a layer of black silver sulfide (Ag₂S) on the surface of the silver article. The overall reaction is 2Ag + H₂S → Ag₂S + H₂.

Silver tarnish is caused by the formation of silver sulfide (Ag₂S) through the reaction of silver with hydrogen sulfide in the air.

H₂S is the reactant from the air. AgS is not the correct chemical formula for silver sulfide (silver has a +1 charge, sulfide has a -2 charge, so Ag₂S is formed). AgSO₄ is silver sulfate, which would typically be formed by a reaction with sulfuric acid or sulfates, not the common cause of black tarnish in air. The black deposit is definitively silver sulfide (Ag₂S).

9. Which one of the following oxides shows both acidic and basic

Which one of the following oxides shows both acidic and basic behaviour?

Zinc oxide

Copper oxide

Magnesium oxide

Calcium oxide

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Amphoteric oxides are oxides that show both acidic and basic properties. They react with acids to form salt and water, and also react with bases to form salt and water. Zinc oxide (ZnO) is a common example of an amphoteric oxide. It reacts with an acid like HCl: ZnO + 2HCl → ZnCl₂ + H₂O, and it reacts with a base like NaOH: ZnO + 2NaOH + H₂O → Na₂[Zn(OH)₄] (sodium tetrahydroxozincate).

Amphoteric oxides can react with both acids and bases.

Copper oxide (CuO), Magnesium oxide (MgO), and Calcium oxide (CaO) are typically basic oxides formed by metals. They react with acids but generally do not react with bases. For instance, MgO + 2HCl → MgCl₂ + H₂O, but MgO does not react with NaOH. Other examples of amphoteric oxides include Al₂O₃, PbO, SnO, etc.

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10. Identify the correct pair of elements among the following which are li

Identify the correct pair of elements among the following which are liquid at room temperature and standard pressure.

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Bromine and fluorine

Mercury and rubidium

Bromine and thallium

Bromine and mercury

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At room temperature (typically 20-25°C) and standard pressure (1 atm), only two elements are liquid: Bromine (Br₂) and Mercury (Hg). Bromine is a reddish-brown volatile liquid that exists as diatomic molecules (Br₂). Mercury is a silvery-white heavy metal.

Bromine and Mercury are the only two elements that exist as liquids under standard conditions of temperature and pressure.

Fluorine (F₂) is a pale yellow gas at room temperature. Rubidium (Rb) is an alkali metal that is solid at room temperature but has a low melting point (39.3 °C). Thallium (Tl) is a metal that is solid at room temperature. Some other elements like Gallium (Ga), Cesium (Cs), and Francium (Fr) melt at temperatures slightly above typical room temperature but are often considered liquids at slightly elevated room temperatures or in warm environments. However, only Bromine and Mercury are liquid under *standard* room temperature conditions.