UPSC NDA-1

21. Which of the following statements with regard to the phenomenon of the

Which of the following statements with regard to the phenomenon of the primary rainbow formation by water droplets is/are correct ?

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  • 1. It involves refraction and one internal reflection of sunlight.
  • 2. It involves refraction of sunlight only.
  • 3. It is formed as the inner bow.
  • 4. It may involve more than one internal reflection as well as refraction of sunlight.

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1 only
1 and 3
3 and 4
2 and 3
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UPSC NDA-1 – 2024
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The formation of a primary rainbow involves sunlight entering a water droplet, undergoing refraction at the front surface, reflecting internally off the back surface, and then refracting again as it exits the front surface. This process disperses the white light into its constituent colors. Statement 1 correctly describes this as involving refraction and one internal reflection. The primary rainbow appears as the inner and brighter bow in the sky, formed by deviation angles around 40-42 degrees relative to the direction opposite the sun. Statement 3 correctly identifies it as the inner bow. Statement 2 is incorrect as reflection is crucial. Statement 4 describes the formation of secondary or higher-order rainbows, not the primary rainbow, which specifically involves a single internal reflection.
– Primary rainbow involves one internal reflection and two refractions of sunlight within water droplets.
– Primary rainbow is the brighter, inner bow observed in the sky.
– Different colors are deviated by different angles due to dispersion during refraction.
The secondary rainbow, which is fainter and appears outside the primary bow (at angles around 50-53 degrees), is formed by sunlight undergoing two internal reflections within the water droplet. The order of colors is reversed in the secondary rainbow compared to the primary one.

22. For a human eye, where u is the distance of an object from the eye, f

For a human eye, where u is the distance of an object from the eye, f is the focal length of the lens and v is the distance of image from the eye, which is the correct schematic graph ?

”[Graph
” option2=”[Graph (v vs u)]” option3=”[Graph (f vs u)]” option4=”[Graph (v vs f)]” correct=”option3″]

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This question was previously asked in
UPSC NDA-1 – 2024
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The correct schematic graph for the human eye shows how the focal length (f) of the lens changes with the distance of the object (u). The distance of the image (v) from the lens to the retina is constant in a healthy eye. The lens formula relating these quantities is 1/f = 1/v + 1/u (using magnitudes for a convex lens forming a real image). Since v is constant, as the object distance u increases, 1/u decreases. For the equality to hold, 1/f must also decrease, which means f must increase. Conversely, as u decreases, 1/u increases, so 1/f must increase, and f must decrease. The human eye’s lens changes its shape (accommodation) to adjust its focal length f to focus objects at different distances u onto the fixed retina distance v. The graph in option C shows f increasing as u increases, which is consistent with the lens formula and the process of accommodation.
– For the human eye, the image distance (v) is fixed (distance to the retina).
– The eye lens changes its focal length (f) to focus objects at different distances (u). This process is called accommodation.
– As the object distance (u) increases, the focal length (f) required to focus the image on the retina increases.
– The lens formula is 1/f = 1/v + 1/u. With v constant, f is a function of u.
The graph in option C shows f increasing with u. The functional relationship derived from the lens formula is f(u) = (v*u) / (v+u). The second derivative of this function with respect to u is negative, indicating the curve is concave down, which matches the shape depicted in graph C. Options A and B are incorrect because they show v varying with u, whereas v is constant. Option D shows v varying with f, which is also incorrect as v is constant.

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23. A microscope may be a combination of:

A microscope may be a combination of:

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two convex lenses.
a convex and a concave lens.
two concave lenses.
a convex lens and a convex mirror.
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A microscope may be a combination of two convex lenses.
A simple microscope consists of a single convex lens. A compound microscope, which provides higher magnification, typically uses at least two lenses: an objective lens and an eyepiece lens. Both the objective lens and the eyepiece lens in a basic compound microscope setup are convex lenses.
The objective lens (convex) forms a real, inverted, and magnified intermediate image of the object. The eyepiece lens (convex) acts as a simple magnifier, forming a virtual, erect (relative to the intermediate image), and further magnified final image that the observer sees. More complex microscope optics may involve combinations of convex and concave lenses or multiple lens elements to correct aberrations, but the fundamental design of a compound microscope relies on two primary convex lenses.

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24. Consider the following electric circuit : [Circuit diagram is present

Consider the following electric circuit :
[Circuit diagram is present in the image]
The current in the above electric circuit is :

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1 A
(10/15) A
2 A
1.5 A
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The current in the electric circuit is (10/15) A.
The circuit consists of a 10V battery connected to two resistors, 5 ohms and 10 ohms, in series. For resistors connected in series, the total resistance (R_total) is the sum of individual resistances. R_total = 5 Ω + 10 Ω = 15 Ω. According to Ohm’s Law, the current (I) flowing through the circuit is given by V = I * R_total, where V is the voltage. Therefore, I = V / R_total = 10 V / 15 Ω = (10/15) A.
The fraction (10/15) can be simplified to (2/3) A. The options provide the unsimplified fraction (10/15) A as option B. If the resistors were in parallel, the total resistance would be calculated differently (1/R_total = 1/R1 + 1/R2).

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25. Of the following, which does not belong to a nuclear reactor ?

Of the following, which does not belong to a nuclear reactor ?

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A turbine
A heat exchanger
A mechanism to reduce CO2 emission
A reaction chamber
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A mechanism to reduce CO2 emission does not belong to a nuclear reactor itself.
Nuclear reactors are power plants that generate electricity through nuclear fission, a process that does not produce carbon dioxide. Therefore, a mechanism *within the reactor* specifically designed to reduce CO2 emissions is not a component of a nuclear reactor. Other listed items (turbine, heat exchanger, reaction chamber/core) are integral parts of the nuclear power generation process.
Nuclear power is often promoted as a low-carbon energy source because it avoids the CO2 emissions associated with burning fossil fuels. However, this is a characteristic of the energy source itself (absence of CO2 production during fission) rather than a component within the reactor designed for CO2 reduction.

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26. If the block P as shown in the figure below were to be at rest, what s

If the block P as shown in the figure below were to be at rest, what should the magnitude of force F be ?

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5 N
6 N
8 N
10 N
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The magnitude of force F should be 10 N.
For block P to be at rest, the net horizontal force on it must be zero. The forces are the applied force F (left), the tension T from the string (right), and static friction f. The tension T is equal to the weight of block Q, T = m_Q * g = 2 kg * g. Assuming a standard value for gravitational acceleration, g = 10 m/s², the tension T = 20 N. Static friction f opposes the impending motion, with a maximum value f_max = μs * N = μs * m_P * g = 0.4 * m_P * g (where N is the normal force equal to the weight of P). Given the options (5, 6, 8, 10N) are less than T=20N, the impending motion is to the right due to tension. Friction must act to the left to help F balance T: F + f = T. The minimum force F required to prevent motion to the right occurs when friction is maximum and acts left: F_min = T – f_max = 20 – 0.4 * m_P * 10 = 20 – 4 * m_P. If we assume the mass of P is m_P = 2.5 kg, then f_max = 0.4 * 2.5 * 10 = 10 N. In this specific scenario, F_min = 20 N – 10 N = 10 N. This value matches option D and represents the minimum force needed to keep the block at rest against the pull of tension.
Without the mass of block P, the problem is technically underspecified for a unique value of F that keeps the block at rest (any F in the range [T – f_max, T + f_max] works). However, the presence of a specific value among the options suggests either an implicit assumption about m_P or that the question is asking for a significant value within the possible range, likely the minimum required force, which is calculable if a specific m_P value is assumed.

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27. Which one among the following cells produces antibodies against a fore

Which one among the following cells produces antibodies against a foreign antigen ?

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Lymphocytes
Erythrocytes
Eosinophils
Platelets
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Lymphocytes produce antibodies against a foreign antigen.
Lymphocytes are a type of white blood cell crucial to the immune system. Specifically, B lymphocytes (B cells) are responsible for producing antibodies. Upon encountering an antigen, B cells differentiate into plasma cells, which are highly specialized in synthesizing and secreting large quantities of specific antibodies to target and neutralize the antigen.
Erythrocytes (red blood cells) transport oxygen. Eosinophils are granulocytes involved in defense against parasites and in allergic reactions. Platelets are involved in blood clotting. None of these cell types produce antibodies.

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28. Bacterial DNA is referred to as naked because it is not associated w

Bacterial DNA is referred to as naked because it is not associated with :

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any scaffold
proteins
ribozymes
plasmid
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Bacterial DNA is referred to as naked because it is not associated with proteins (specifically histone proteins).
The term “naked DNA” in the context of bacteria refers to the fact that their DNA is not complexed with histone proteins to form chromatin, as is the case with eukaryotic DNA. While bacterial DNA does associate with some non-histone proteins for packaging and organization, the absence of histones is the defining characteristic leading to the term “naked”.
Eukaryotic DNA is wrapped around histone proteins to form nucleosomes, which are further folded to form chromatin. This structured packaging is absent in prokaryotes. Plasmids are extra-chromosomal DNA found in bacteria, not proteins associated with the main chromosome.

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29. In most prokaryotes, the chromosome number is :

In most prokaryotes, the chromosome number is :

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4
3
2
1
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In most prokaryotes, the chromosome number is 1.
Prokaryotic organisms, such as bacteria and archaea, typically possess a single, circular chromosome located in a region called the nucleoid within the cytoplasm. Unlike eukaryotes, their genetic material is not enclosed within a membrane-bound nucleus and is usually present as one primary chromosome.
While most prokaryotes have one main circular chromosome, some may have linear chromosomes or additional smaller circular DNA molecules called plasmids. Eukaryotes, in contrast, have multiple linear chromosomes located within the nucleus.

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30. Leaves of most plants appear green because the chlorophyll present in

Leaves of most plants appear green because the chlorophyll present in it :

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absorbs red and blue light while reflecting green light.
absorbs green light only.
absorbs green light while reflecting red and blue light.
reflects red light and absorbs blue and green light.
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Leaves of most plants appear green because the chlorophyll present in it absorbs red and blue light while reflecting green light.
Chlorophyll is the primary pigment in plants responsible for photosynthesis. It absorbs light most strongly in the blue and red regions of the visible spectrum. The green light from the spectrum is not absorbed efficiently; instead, it is reflected by the leaves. This reflected green light is what our eyes perceive, making the leaves appear green.
The absorption spectrum of chlorophyll shows peaks in the blue (~430 nm) and red (~662 nm) wavelengths and a trough in the green (~550 nm) wavelength, corresponding to reflection. The energy from the absorbed red and blue light is used for photosynthesis.

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