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Physics Assertion (A) & Reason (B) MCQ

Electromagnetic Waves · Gujarat Board (GSEB) STD 12 Science (GSEB - English Medium). 14 questions.

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Assertion (A) & Reason (B) MCQ

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14 questions · timed · auto-graded

Question 11 Mark
Two statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below.
  1. Both A and R are true and R is the correct explanation of A.
  2. Both A and R are true but R is not the correct explanation of A.
  3. A is true but R is false.
  4. A is false and R is also false.
Assertion: X-rays in vacuum travel faster than light waves in vacuum
Reason: The energy of X-rays photon is less than that of light photon.
Answer
  1. A is false and R is also false.

Explanation:

All electromagnetic waves including X-rays travels with same velocity in vacuum. The energy of X-rays is greater than energy of the light because energy is inversely proportional to wavelength $\big(\text{E}=\frac{\text{hc}}{\lambda}\big)$ and wavelength of X-rays are smaller than light waves.

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Question 21 Mark
Two statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below.
  1. Both A and R are true and R is the correct explanation of A.
  2. Both A and R are true but R is not the correct explanation of A.
  3. A is true but R is false.
  4. A is false and R is also false.
Assertion: Electromagnetic waves exert pressure called radiation pressure.
Reason: Electromagnetic waves carries energy.
Answer
  1. Both A and R are true but R is not the correct explanation of A.

Explanation:

Electromagnetic waves transport linear momentum as well as energy. When electromagnetic waves strike a surface, a pressure is exerted on the surface. If the intensity of wave is I, the radiation pressure P (force per unit area) exerted on the perfectly absorbing surface is $\text{p}=\frac{\text{I}}{\text{c}}.$

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Question 31 Mark
Two statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below.
  1. Both A and R are true and R is the correct explanation of A.
  2. Both A and R are true but R is not the correct explanation of A.
  3. A is true but R is false.
  4. A is false and R is also false.
Assertion: The microwaves are better carriers of signals than radio wave.
Reason: The electromagnetic waves do not required any material medium for propagation.
Answer
  1. Both A and R are true but R is not the correct explanation of A.

Explanation:

Microwaves are the electromagnetic waves of wavelength of the order of a few millimetres, which is less than those of T. V. signals. On account of smaller wavelength, the microwaves can be transmitted as beam signals in a particular direction and are much better than radiowaves because microwaves do not spread or bend around the corners of any obstacle coming in their way. Therefore microwaves are better carriers of signals than radiowaves.

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Question 41 Mark
Two statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below.
  1. Both A and R are true and R is the correct explanation of A.
  2. Both A and R are true but R is not the correct explanation of A.
  3. A is true but R is false.
  4. A is false and R is also false.
Assertion: In an electromagnetic wave, magnitude of magnetic field vector is much smaller than the magnitude of electric field vector.
Reason: Energy of electromagnetic waves is shared equally by the electric and magnetic fields.
Answer
  1. Both A and R are true but R is not the correct explanation of A.

Explanation:

At every instant the ratio of the magnitudes of the electric field to the magnetic field of an em wave is given by $\frac{\text{E}}{\text{B}}=\text{c}.$

From this equation, the magnitude of electric vector is much greater than the magnitude of magnetic vector. Also electromagnetic waves carry energy which is equally shared by electric and magnetic field.

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Question 51 Mark
Two statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below.
  1. Both A and R are true and R is the correct explanation of A.
  2. Both A and R are true but R is not the correct explanation of A.
  3. A is true but R is false.
  4. A is false and R is also false.
Assertion: Infrared waves sometimes referred as heat waves.
Reason: Infrared waves heat up the earth surface.
Answer
  1. Both A and R are true but R is not the correct explanation of A.

Explanation:

Infrared waves are sometimes called heat waves. This is because water molecules present in most materials readily absorb infrared waves. After absorption, their thermal motion increases, that is, they heat up and heat their surroundings.

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Question 61 Mark
Two statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below.
  1. Both A and R are true and R is the correct explanation of A.
  2. Both A and R are true but R is not the correct explanation of A.
  3. A is true but R is false.
  4. A is false and R is also false.
Assertion: Radio waves cannot be diffracted by the buildings.
Reason: The wavelength of radio waves is very small.
Answer
  1. A is false and R is also false.

Explanation:

For wave to suffer diffraction, the wavelength should be of the order of size of the obstacle. The wavelength of radio waves (short radio waves) is order of the size of the building and the other obstacles coming in their path and hence they easily get diffracted.

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Question 71 Mark
Directions: In the following questions, the Assertions (A) and Reason(s) (R) have been put forward. Read both the statements carefully and choose the correct alternative from the following:
Assertion: When variable frequency AC source is connected to a capacitor, displacement current increases with increase in frequency.
Reason: As frequency increases conduction current also increases.
  1. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
  2. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
  3. Assertion is correct but Reason is incorrect.
  4. Both Assertion and Reason are incorrect.
Answer
  1. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.

Explanation:

Increase in frequency causes decrease in impedance of the capacitor and consequent increase in current which equals displacement current between the plates of capacitor.

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Question 81 Mark
Two statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below.
  1. Both A and R are true and R is the correct explanation of A.
  2. Both A and R are true but R is not the correct explanation of A.
  3. A is true but R is false.
  4. A is false and R is also false.
Assertion: Light is a transverse wave but not an electromagnetic wave.
Reason: Maxwell showed that speed of electromagnetic waves is related to the permittivity of the medium through which it travels.
Answer
  1. A is false and R is also false.

Explanation:

The speed of em waves in free space is given by $\text{c}=\frac{1}{\sqrt{\mu_0\in_0}}.$

Where $\mu_0=4\pi\times10^{-7}\text{N}\frac{\text{s}^2}{\text{C}^2}$ is penneability constant of vacuum and $\in_0=8.85419\times10^{-12}\frac{\text{C}^2}{\text{N}}\text{m}^2$ is the permittivity of free space. After substituting these value, the value of $\text{c}=\big(2.99792\times10^8\frac{\text{m}^2}{\text{s}}\big)$ which is same as the speed of light in vacuum. From this it is concluded that light is an electromagnetic wave.

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Question 91 Mark
Two statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below.
  1. Both A and R are true and R is the correct explanation of A.
  2. Both A and R are true but R is not the correct explanation of A.
  3. A is true but R is false.
  4. A is false and R is also false.
Assertion: X-ray astronomy is possible only from satellites orbiting the earth.
Reason: Efficiency of X-rays telescope is large as compared to any other telescope.
Answer
  1. A is true but R is false.

Explanation:

The earth's atmosphere is transparent to visible tight and radio waves, but absorbs X-rays. Therefore X-rays telescope cannot be used on earth surface.

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Question 101 Mark
Two statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below.
  1. Both A and R are true and R is the correct explanation of A.
  2. Both A and R are true but R is not the correct explanation of A.
  3. A is true but R is false.
  4. A is false and R is also false.
Assertion: Light can travel in vacuum whereas sound cannot do so.
Reason: Light has an electromagnetic wave nature whereas sound is mechanical wave.
Answer
  1. Both A and R are true and R is the correct explanation of A.

Explanation:

Light being electromagnetic wave do not require any material medium for its propagation. Hence light can travel in vacuum. On the other hand sound is a mechanical wave and requires a material medium for its propagation. Hence sound cannot travel in vacuum.

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Question 111 Mark
Two statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below.
  1. Both A and R are true and R is the correct explanation of A.
  2. Both A and R are true but R is not the correct explanation of A.
  3. A is true but R is false.
  4. A is false and R is also false.
Assertion: The electric field and magnetic field have equal average values in linearly polarised plane em wave.
Reason: The electric energy and magnetic energy have equal average values in linearly polarised plane em wave.
Answer
  1. Both A and R are true but R is not the correct explanation of A.

Explanation:

In case of a linearly polarised plane electromagnetic wave, the average values of electric field and magnetic field are equal and average values of electric energy and magnetic energy are also equal.

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Question 121 Mark
Two statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below.
  1. Both A and R are true and R is the correct explanation of A.
  2. Both A and R are true but R is not the correct explanation of A.
  3. A is true but R is false.
  4. A is false and R is also false.
Assertion: When a charged particle moves in a circular path. It produces electromagnetic wave.
Reason: Charged particle has acceleration.
Answer
  1. Both A and R are true and R is the correct explanation of A.

Explanation:

Accelerated charges radiate electromagnetic waves.

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Question 131 Mark
Two statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below.
  1. Both A and R are true and R is the correct explanation of A.
  2. Both A and R are true but R is not the correct explanation of A.
  3. A is true but R is false.
  4. A is false and R is also false.
Assertion: Velocity of light is constant in all media.
Reason: Light is an electromagnetic wave which has constant velocity in all media.
Answer
  1. A is false and R is also false.

Explanation:

Velocity of light has different values in different media.

It depends on the refractive index of the medium.

Related by formula $\text{v}_\text{medium}=\frac{\text{Velocity in vacuum}}{\text{Refractive index of medium }}$

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Question 141 Mark
Two statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below.
  1. Both A and R are true and R is the correct explanation of A.
  2. Both A and R are true but R is not the correct explanation of A.
  3. A is true but R is false.
  4. A is false and R is also false.
Assertion: The electromagnetic waves are transverse in nature.
Reason: These waves propagate in straight lines.
Answer
  1. Both A and R are true but R is not the correct explanation of A.

Explanation:

The electromagnetic waves consist of sinusoidally time varying electric and magnetic fields acting at right angles to each other as well as at right angles to the direction of propagation of waves i.e., electromagnetic waves are transverse in nature.

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