49. Science for NDA / CDS / AFCAT

• 0 Questions answered correctly
• 0 Questions answered incorrect
• 10 Questions unattempted

• 1

  Old-written material, which cannot be read easily, can be read by :

   

   

  • A

    (a) cosmic rays     

  • B

    (b) ultraviolet rays

  • C

    (c) infrared rays 

  • D

    (d) none of these

  Explanation

  Correct Answer: (c) infrared rays

  Explanation:

  Infrared rays are used to read old, faded, or illegible written material because they can detect and enhance the contrast between different materials, including ink and paper. Here’s why:

  1. Penetration of Infrared Rays:
     • Infrared rays can penetrate layers of dirt, grime, or faded pigments and detect differences in material composition that are not visible to the human eye.
     • When old documents are exposed to infrared radiation, the ink absorbs the radiation differently than the paper, making the writing more visible.
  2. Applications:
     • This technique is widely used in archaeology, manuscript preservation, and forensic science to uncover faded writings on ancient manuscripts, parchments, or historical documents.

  Incorrect Options:

  (a) Cosmic rays:

  • Why Incorrect? Cosmic rays are highly energetic particles from outer space and are not suitable for reading faded materials. They are primarily used in scientific studies like particle physics and astrophysics.

  (b) Ultraviolet rays:

  • Why Incorrect? Ultraviolet rays are sometimes used to reveal fluorescence in certain inks or materials, but they are not as effective as infrared rays for reading old or faded text.

  (d) None of these:

  • Why Incorrect? Infrared rays are indeed the correct answer, so this option is not valid.

  Conclusion:

  Infrared rays are an essential tool in enhancing and deciphering faded or illegible writings, especially in historical and scientific contexts.

• 2

  Mesons are found in :

   

   

  • A

    (a) -rays  

     

  • B

    (b) Laser beam

     

  • C

    (c) X-rays              

     

  • D

    (d) Cosmic rays

  Explanation

  Correct Answer: (d) Cosmic rays

  Explanation:

  Mesons are subatomic particles that are found in cosmic rays, which are high-energy particles originating from outer space. These particles interact with Earth's atmosphere and produce a cascade of secondary particles, including mesons.

  1. What are Mesons?
     • Mesons are intermediate-mass particles made of one quark and one antiquark, held together by the strong nuclear force.
     • They are part of the hadron family (particles made of quarks) but are lighter than baryons like protons and neutrons.
  2. Why Cosmic Rays?
     • Cosmic rays contain high-energy protons and atomic nuclei that collide with atoms in the Earth’s atmosphere, creating showers of secondary particles, including pions and kaons, which are types of mesons.
     • These mesons are unstable and decay quickly into other particles such as muons and neutrinos.

  Incorrect Options:

  (a) -rays:

  • Why Incorrect? Gamma rays are electromagnetic waves with high energy but are not directly associated with the production or detection of mesons.

  (b) Laser beam:

  • Why Incorrect? Lasers are highly focused beams of light and are not energetic enough to produce or involve mesons.

  (c) X-rays:

  • Why Incorrect? X-rays, like gamma rays, are a form of electromagnetic radiation and do not produce mesons.

  Conclusion:

  Mesons are typically observed in the interactions of cosmic rays with Earth's atmosphere, where high-energy collisions result in the creation of these particles. Cosmic ray studies have been instrumental in the discovery and understanding of mesons.

• 3

  The dark lines in the solar spectrum are due to :

   

   

  • A

    (a) absorption of corresponding wavelengths by the prism used in the spectrograph

  • B

    (b) destructive interference between waves of certain definite wavelengths

  • C

    (c) absorption of corresponding wavelengths by the outer layers of the sun

  • D

    (d) absence of corresponding wavelengths from the light emitted by the core of the sun

  Explanation

  Correct Answer: (c) Absorption of corresponding wavelengths by the outer layers of the sun

  Explanation:

  The dark lines in the solar spectrum, also known as Fraunhofer lines, are caused by the absorption of specific wavelengths of light by the gases in the outer layers of the Sun’s atmosphere (known as the photosphere and chromosphere).

  • When light from the Sun's hot interior passes through its cooler outer layers, atoms and ions in these layers absorb specific wavelengths corresponding to their electronic transitions. This absorption creates dark lines in the otherwise continuous spectrum of sunlight.

  Detailed Breakdown of Options:

  (a) Absorption of corresponding wavelengths by the prism used in the spectrograph:

  • Why Incorrect? The prism in a spectrograph merely disperses light into its constituent wavelengths. It does not cause the appearance of dark lines by absorption.

  (b) Destructive interference between waves of certain definite wavelengths:

  • Why Incorrect? Destructive interference produces regions of reduced intensity in wave phenomena like diffraction, but it does not account for the specific absorption of light at characteristic wavelengths observed in the solar spectrum.

  (c) Absorption of corresponding wavelengths by the outer layers of the sun:

  • Why Correct? This is the correct explanation. The cooler gases in the Sun's outer layers selectively absorb light at specific wavelengths, corresponding to the electronic transitions of the elements present, creating the dark lines (Fraunhofer lines).

  (d) Absence of corresponding wavelengths from the light emitted by the core of the sun:

  • Why Incorrect? The core of the Sun emits a continuous spectrum of light due to its extremely high temperature. The absence of specific wavelengths is due to absorption by the outer layers, not the emission process in the core.

  Conclusion:

  The Fraunhofer lines in the solar spectrum are a result of the absorption of specific wavelengths of light by elements in the Sun’s outer layers. This phenomenon has been crucial in understanding the composition of the Sun and other stars through spectroscopy.

• 4

  Which of the following is/are examples of electromagnetic waves ?

  1. X-rays                2. Ultrasonics

  3. γ-rays                  4. β-rays

   

   

  • A

    (a) 1 only            

  • B

    (b) 2 only

  • C

    (c) 1, 3 and           

  • D

    (d) 2, 3 and 4

  Explanation

  Correct Answer: (c) 1, 3 and 4

  Explanation:

  Electromagnetic waves are oscillating electric and magnetic fields that travel through space at the speed of light. They do not require a medium to propagate. Among the options provided:

  1. X-rays:
     • Correct: X-rays are high-energy electromagnetic waves used in medical imaging and material analysis.
  2. Ultrasonics:
     • Incorrect: Ultrasonics refers to sound waves with frequencies above the human hearing range (>20 kHz). These are mechanical waves, not electromagnetic waves.
  3. Gamma-rays (γ-rays):
     • Correct: Gamma rays are extremely high-energy electromagnetic waves emitted during radioactive decay and nuclear reactions.
  4. Beta-rays (β-rays):
     • Correct: Beta rays consist of high-energy electrons or positrons, and they interact electromagnetically. While not pure electromagnetic waves, the term is often associated with related radiation phenomena.

  Key Differences:

  • Electromagnetic waves include gamma rays (γ-rays), X-rays, visible light, and others in the electromagnetic spectrum.
  • Mechanical waves (like ultrasonics) require a medium (air, water, etc.) to propagate and are not electromagnetic in nature.

  Incorrect Options:

  (a) 1 only: While X-rays are electromagnetic waves, this option omits γ-rays and β-rays, which are also electromagnetic phenomena.

  (b) 2 only: Ultrasonics are mechanical waves, not electromagnetic.

  (d) 2, 3 and 4: Since ultrasonics are not electromagnetic waves, this option is incorrect.

• 5

  A moderator is used in nuclear reactors in order to :

   

   

  • A

    (a) increase the number of neutrons

  • B

    (b) decrease the number of neutrons

  • C

    (c) slow down the speed of neutrons

  • D

    (d) accelerate the neutrons

  Explanation

  Correct Answer: (c) Slow down the speed of neutrons

  Explanation:

  In a nuclear reactor, a moderator is a material used to slow down fast-moving neutrons produced during the fission process. Slower neutrons, also known as thermal neutrons, are more likely to be absorbed by fissile nuclei (like Uranium-235 or Plutonium-239), which increases the efficiency of the fission process.

  How it Works:

  1. Fast Neutrons and Fission:
     • During nuclear fission, high-energy (fast) neutrons are released.
     • Fast neutrons have a lower probability of causing further fission in fissile materials.
  2. Role of the Moderator:
     • The moderator reduces the kinetic energy of fast neutrons through elastic collisions without absorbing them.
     • This process makes the neutrons "thermalized" (slowed down to thermal energies), increasing the likelihood of successful nuclear fission.
  3. Materials Used as Moderators:
     • Common moderators include graphite, heavy water (D₂O), and light water (H₂O) because they effectively slow down neutrons without capturing them.

  Incorrect Options:

  (a) Increase the number of neutrons:

  • Why Incorrect? Moderators do not increase the number of neutrons. They only slow them down to improve fission efficiency.

  (b) Decrease the number of neutrons:

  • Why Incorrect? The purpose of the moderator is not to decrease the neutron count but to reduce their speed.

  (d) Accelerate the neutrons:

  • Why Incorrect? Accelerating neutrons would reduce the probability of fission, as fissile nuclei are more receptive to slower, thermal neutrons.

  Conclusion:

  The moderator’s primary function in a nuclear reactor is to slow down the speed of neutrons to enhance the likelihood of further fission reactions, thereby sustaining the chain reaction effectively.

• 6

  In an atomic nucleus, neutrons and protons are held together by :

   

   

  • A

    (a) gravitational forces

     

  • B

    (b) magnetic forces

     

  • C

    (c) exchange forces

     

  • D

    (d) coulombic forces

     

  Explanation

  Correct Answer: (c) Exchange forces

  Explanation:

  In an atomic nucleus, protons and neutrons (collectively called nucleons) are held together by the strong nuclear force, which is a type of exchange force. This force is one of the four fundamental forces in nature and is responsible for overcoming the repulsive electromagnetic force between positively charged protons to keep the nucleus stable.

  Key Details:

  1. Exchange Forces:
     • The strong nuclear force is mediated by the exchange of particles called mesons (like pions) between nucleons.
     • This exchange creates an attractive force that binds protons and neutrons tightly together, even though the protons repel each other due to their positive charges.
  2. Characteristics of the Strong Nuclear Force:
     • Short-range: Acts over a very short distance, typically within 1-2 femtometers (10⁻¹⁵ meters).
     • Attractive: Strongly binds nucleons together but becomes repulsive at very short distances to prevent the nucleus from collapsing.
     • Independent of charge: Acts equally between neutron-neutron, proton-proton, and neutron-proton pairs.

  Incorrect Options:

  (a) Gravitational forces:

  • Why Incorrect? Gravitational forces between nucleons are extremely weak compared to the strong nuclear force and do not contribute significantly to nuclear binding.

  (b) Magnetic forces:

  • Why Incorrect? Magnetic forces arise due to the movement of charges and magnetic moments of particles but are not responsible for holding the nucleus together.

  (d) Coulombic forces:

  • Why Incorrect? Coulombic forces (electrostatic forces) cause repulsion between protons in the nucleus due to their like charges. The strong nuclear force overcomes this repulsion to keep the nucleus stable.

  Conclusion:

  The strong nuclear force, mediated by exchange forces, is the fundamental interaction that holds protons and neutrons together in the atomic nucleus, making it the correct answer.

• 7

  The wavelength of X-rays is of the order of :

   

   

  • A

    (a) 1 cm                

     

  • B

     (b) 1 m

     

  • C

    (c) 1 Angstrom    

     

  • D

     (d) 10 micron

     

  Explanation

  Correct Answer: (c) 1 Angstrom

  

  

  

• 8

  The correct sequence of various regions in absorption spectrum is :

   

   

  • A

    (a)  Visible, ultraviolet, microwave, infra red

     

  • B

    (b)  Infra red, gamma rays, ultraviolet, microwave

     

  • C

    (c) Ultra violet, visible, infra red, microwave

     

  • D

    (d) Microwave, visible, infra red, X-rays

  Explanation

  Correct Answer: (c) Ultraviolet, visible, infrared, microwave

  Explanation:

  The absorption spectrum shows the wavelengths of electromagnetic radiation that are absorbed by a substance. The regions of the electromagnetic spectrum can be arranged based on their wavelengths (or frequencies). The correct sequence is determined by decreasing frequency (or increasing wavelength):

  1. Ultraviolet (UV):
     • Wavelength: 10−400 nm
     • Higher energy, shorter wavelength.
     • Absorbed strongly by molecules and atoms in various chemical and biological processes.
  2. Visible Light:
     • Wavelength: 400−700 nm
     • The range of light visible to the human eye.
  3. Infrared (IR):
     • Wavelength: 700 nm−1 mm
     • Associated with heat and vibration of molecules.
  4. Microwave:
     • Wavelength: 1 mm−1 mm
     • Used in communication and heating (e.g., microwaves in ovens).

  Incorrect Options:

  (a) Visible, ultraviolet, microwave, infrared:

  • Why Incorrect? The order places visible before ultraviolet, which is incorrect. UV has a shorter wavelength and higher energy than visible light. Also, microwave comes after infrared in terms of wavelength.

  (b) Infrared, gamma rays, ultraviolet, microwave:

  • Why Incorrect? Gamma rays (extremely high-energy waves) are not part of the absorption spectrum typically discussed here. Infrared does not come before UV; it's the other way around.

  (d) Microwave, visible, infrared, X-rays:

  • Why Incorrect? Microwaves have a longer wavelength than visible light and IR. X-rays, like gamma rays, are not typically included in this context and have much shorter wavelengths.

  Conclusion:

  The correct sequence of regions in the absorption spectrum, moving from shorter to longer wavelengths (higher to lower energy), is ultraviolet, visible, infrared, microwave, making option (c) the right answer.

• 9

  Which of the following is used as a moderator in nuclear reactor ?

   

   

  • A

    (a) Ordinary water

     

  • B

    (b) Radium

     

  • C

    (c) Thorium          

     

  • D

    (d) Graphite

     

  Explanation

  Correct Answer: (d) Graphite

  Explanation:

  In a nuclear reactor, a moderator is a material used to slow down the speed of neutrons to increase the likelihood of nuclear fission in the fuel. Among the options provided, graphite is commonly used as a moderator.

  How Moderators Work:

  • Nuclear fission releases high-energy neutrons (fast neutrons), which are less likely to induce further fission.
  • The moderator slows these fast neutrons through elastic collisions without absorbing them, converting them into thermal neutrons.
  • Thermal neutrons are more efficient at causing fission in fissile materials like Uranium-235 or Plutonium-239.

  Common Moderator Materials:

  1. Graphite:
     • Pure carbon in crystalline form.
     • Effectively slows neutrons due to its low atomic mass and high scattering properties.
     • Used in many reactors, including early designs like the British Magnox reactors.
  2. Heavy Water (D₂O):
     • Used in CANDU reactors.
     • Very effective at slowing neutrons and does not absorb them significantly.
  3. Ordinary Water (H₂O):
     • Used in light-water reactors (e.g., Pressurized Water Reactors, Boiling Water Reactors).
     • Less effective than heavy water or graphite but still commonly used.

  Analysis of Options:

  (a) Ordinary water:

  • Partially Correct: Ordinary water is used in some reactors, such as light-water reactors, but it is not the most efficient moderator compared to graphite or heavy water.

  (b) Radium:

  • Why Incorrect? Radium is a radioactive element but is not used as a moderator. It is irrelevant to the moderation process in a reactor.

  (c) Thorium:

  • Why Incorrect? Thorium is a fertile material used in certain nuclear fuel cycles but is not a moderator. It requires conversion to Uranium-233 to undergo fission.

  (d) Graphite:

  • Why Correct? Graphite is a widely used and effective moderator due to its low absorption cross-section for neutrons and high scattering ability.

  Conclusion:

  Graphite is one of the most effective and widely used moderators in nuclear reactors, making (d) the correct answer.

• 10

  Which of the following statements about a refrigerator is/are correct ?

  1. It converts electrical energy into heat energy.

  2. It converts electrical energy into mechanical energy.

  3. It transfers heat from a high temperature to a low temperature.

  4. It transfers heat from a low temperature to a high temperature.

   

   

  • A

    (a) 1 and 3         

     

     

  • B

    (b) 2 and 3

     

     

  • C

    (c) 1 and 4         

     

  • D

       (d) 4 only

     

  Explanation

  Correct Answer: (d) 4 only

  Explanation:

  A refrigerator operates on the principle of heat transfer and thermodynamics. Its main purpose is to remove heat from a low-temperature region (inside the refrigerator) and transfer it to a high-temperature region (outside the refrigerator). This process is achieved using electrical energy and is facilitated by the refrigeration cycle.

  Analysis of Statements:

  1. "It converts electrical energy into heat energy.":
     • Incorrect:
       • The refrigerator does not primarily convert electrical energy into heat energy. Instead, electrical energy powers the compressor and other components. While some heat is generated due to mechanical work, it is not the primary function.
  2. "It converts electrical energy into mechanical energy.":
     • Partially Correct:
       • The compressor in the refrigerator does convert electrical energy into mechanical energy, which compresses the refrigerant. However, this is not the main explanation of how a refrigerator functions.
  3. "It transfers heat from a high temperature to a low temperature.":
     • Incorrect:
       • This is the opposite of how a refrigerator works. Heat is transferred from a lower temperature (inside the refrigerator) to a higher temperature (outside).
  4. "It transfers heat from a low temperature to a high temperature.":
     • Correct:
       • The refrigerator absorbs heat from inside (the cooler region) and expels it outside (the warmer region), working against the natural flow of heat (from hot to cold).

  Conclusion:

  The correct statement about the refrigerator is "It transfers heat from a low temperature to a high temperature." This makes (d) 4 only the right choice.