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Physics Assertion (A) & Reason (B) MCQ

Wave Optics · Jharkhand Board (JAC) STD 12 Science (Jharkhand - English Medium). 20 questions.

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Assertion (A) & Reason (B) MCQ

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20 questions · 2 auto-graded MCQ + 18 self-marked written.

MCQ 11 Mark
For question 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.
Assertion $(A):$Two point coherent sources of light $S_1$ and $S_2$ are placed on a line as shown. $P$ and $Q$ are two points on that line. If at point $P$ maximum intensity is observed then maximum intensity should also be observed at $Q.$


Reason $(R):$ In the figure of assertion the distance $\left|S_1 P-S_2 P\right|$ is equal to distance $\left|S_2 Q-S_1 Q\right|$.
  • A
    Both $A$ and $R$ are true and $R$ is the correct explanation of $A.$
  • ✓
    Both $A$ and $R$ are true but $R$ is not the correct explanation of $A.$
  • C
    $A$ is true but $R$ is false.
  • D
    $A$ is false and $R$ is also false.
Answer
Correct option: B.
Both $A$ and $R$ are true but $R$ is not the correct explanation of $A.$
If maximum intensity is observed at $P$ then for maximum intensity to be also observed at $Q,S_1$ and $S_2$ must have phase difference of $2 \mathrm{~m} \pi ($where m is an integer$)$.
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Question 21 Mark
For question 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 (A): When tiny circular obstacle is placed in the path of light from some distance, a bright spot is seen at the centre of the shadow of the obstacle.
Reason (R): Destructive interference occurs at the centre of the shadow.
Answer
  1. A is true but R is false.
Explanation:

The waves diffracted from the edges of circular obstacle, placed in the path of light, interfere constructively at the centre of the shadow resulting in the formation of a bright spot.
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Question 31 Mark
For question 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 (A): In interference all the fringes are of same width.
Reason (R): In interference fringe width is independent of position of the fringe.
Answer
  1. Both A and R are true and R is the correct explanation of A.
Explanation:

As given in the expression $\beta=\frac{\text{D}\lambda}{\text{d}}$, fringe width $\beta$ is independent of n (position of fringe), hence all the fringes are of same width.
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Question 41 Mark
For question 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 (A): The film which appears bright in reflected system will appear dark in the transmitted light and vice-versa.
Reason (R): The conditions for film to appear bright or dark in reflected light are just reverse to those in the transmitted light.
Answer
  1. Both A and R are true and R is the correct explanation of A.
Explanation:

For reflected system of the film, the maxima or constructive interference is $2\mu\text{t}\cos\text{r}=$ while the maxima for transmitted system of film is given by equation

$2\mu\text{t}\cos\text{r}=\text{n}\lambda$

where t is thickness of the film and r is angle of refraction.

From these two equations we can see that condition for maxima in reflected system and transmitted system are just opposite.
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Question 51 Mark
For question 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 (A): In Young's experiment the fringe width is directly proportional to wavelength of the source used.
Reason (R): When a thin transparent sheet is placed in front of both the slits of Young's experiment, the fringe width will increase.
Answer
  1. A is true but R is false.
Explanation:

Fringe width $\beta=\frac{\lambda\text{D}}{\text{d}}$ shall remain the same as the waves travel in air only, after passing through the thin transparent sheet. Due to introduction of thin sheet, only path difference of the wave is changed due to which th ere is shift of position of fringes only, which is given as $\triangle\text{x}=\frac{\text{D}(\mu-1)\text{t}}{\text{d}}$, where $\mu$ is refractive index of thin sheet and tis its thickness.
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MCQ 61 Mark
For question 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.
Assertion $(A):$ In Young's double slit experiment the two slits are at distance d apart. Interference pattern is observed on a screen at distance $D$ from the slits. At a point on the screen when it is directly opposite to one of the slits, a dark fringe is observed. Then the wavelength of wave is proportional to square of distance of two slits.
Reason $(R):$ For a dark fringe intensity is zero.
  • A
    Both $A$ and $R$ are true and $R$ is the correct explanation of $A.$
  • ✓
    Both $A$ and $R$ are true but $R$ is not the correct explanation of $A.$
  • C
    $A$ is true but $R$ is false.
  • D
    $A$ is false and $R$ is also false.
Answer
Correct option: B.
Both $A$ and $R$ are true but $R$ is not the correct explanation of $A.$
For case when dark fringe is observed opposite to one of the slit.
Here $S_1P = D$ and $S_2P =\sqrt{\text{D}^2+\text{d}^2}=\text{D}\Big[1+\frac{\text{d}^2}{2\text{D}^2}\Big]$
Path difference $=\text{S}_2\text{P}-\text{S}_1\text{P}=\frac{\text{d}^2}{2\text{D}}=\frac{\lambda}{2}$ or, $\lambda=\frac{\text{d}^2}{\text{D}}$.
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Question 71 Mark
For question 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 (A): When a light wave travels from a rarer to a denser medium, it loses speed. TI1e reduction in speed imply a reduction in energy carried by the light wave.
Reason (R): The energy of a wave is proportional to velocity of wave.
Answer
  1. A is false and R is also false.
Explanation:

When a light wave travel from a rarer to a denser medium it loses speed, but energy carried by the wave does not depend on its speed. Instead, it depends on the amplitude of wave. The frequency also remain constant.
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Question 81 Mark
For question 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 (A): A narrow pulse of light is sent through a medium. The pulse will retain its shape as it travels through the medium.
Reason (R): A narrow pulse is made of harmonic waves with a large range of wavelengths.
Answer
  1. A is true but R is false.
Explanation:

A narrow pulse is made of harmonic waves with a large range of wavelengths. As speed of propagation is different for different wavelengths, the pulse cannot retain its shape while travelling through the medium.
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Question 91 Mark
For question 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 (A): ln YDSE bright and dark fringe are equally spaced.
Reason (R): It only depends upon phase difference.
Answer
  1. A is true but R is false.
Explanation:

Fringe width is given by, $\beta=\frac{\text{D}\lambda}{\text{d}}$

where, D = distance between slit and screen, d = distance between coherent sources of light and $\lambda$ = wavelength of incident light.
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Question 101 Mark
For question 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 (A): Interference obeys the law of conservation of energy.
Reason (R): The energy is redistributed in case of interference.
Answer
  1. Both A and R are true and R is the correct explanation of A.
Explanation:

ln case of interference, intensity of maxima is $\text{I}_\text{max}=\Big(\sqrt{\text{I}_\text{a}}+\sqrt{\text{I}_\text{b}}\Big)^2$ and intensity of minima is $\text{I}_\text{min}=\Big(\sqrt{\text{I}_\text{a}}-\sqrt{\text{I}_\text{b}}\Big)^2$. Thus, whatever energy disappears at the minimum is actually appearing at the maximum. So, the law of conservation of energy holds good in the phenomenon of interference because in interference energy is neither created nor destroyed but is redistributed.
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Question 111 Mark
For question 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 (A): When monochromatic light is incident on a surface separating two media, the reflected and refracted light both have the same frequency as the incident frequency.
Reason (R): The frequency of monochromatic light depends on media.
Answer
  1. A is true but R is false.
Explanation:

The reflection and refraction of light occurs on account of interaction of light with the atoms of the surface of separation. These atoms can be regarded as oscillators.

Light incident on the interface forces the atomic oscillators to oscillate with frequency of incident light. As frequency of light emitted by these (charged) oscillators is equal to theirown frequency ofosci Ration, therefore, reflected and refracted light have the same frequency as that of incident light.
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Question 121 Mark
For question 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 (A): When a tiny circular obstacle is placed in the path of light from a distant source, a bright spot is seen at the centre of the shadow of the obstacle.
Reason (R): Wave diffracted from the edges of circular obstacle interfere constructively at the centre of the shadow resulting in the formation of bright spot.
Answer
  1. Both A and R are true and R is the correct explanation of A.
Explanation:

Point of constructive interference or maxima is obtained when path difference between the two interfering waves is an integral multiple of wavelength.
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Question 131 Mark
For question 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 (A): We cannot get diffraction pattern from a wide slit illuminated by monochromatic light.
Reason (R): In diffraction pattern, all the bright bands are not of the same intensity.
Answer
  1. Both A and R are true but R is NOT the correct explanation of A.
Explanation:

When slit is wide $(\text{i.e., a}>>\lambda)$, bending of light becomes so small that it cannot be detected upto a certain distance of screen from the slit. Hence, practically, no diffraction occurs.
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Question 141 Mark
For question 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 (A): Newton's rings are formed in the reflected system. When the space between the lens and the glass plate is filled with a liquid of refractive index greater than that of glass, the central spot of the pattern is dark.
Reason (R): The reflections in Newton's ring cases will be from a denser to a rarer medium and the two interfering rays are reflected under similar conditions.
Answer
  1. Both A and R are true and R is the correct explanation of A.
Explanation:

The central spot of Newton's rings is dark when the medium between piano convex lens and plane glass is rarer than the medium oflens and glass. The central spot is dark because the phase change of $\pi$ is introduced between the rays reflected from surfaces of denser to rarer and rarer to denser media.
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Question 151 Mark
For question 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 (A): The interference pattem is observed when source is monochromatic and coherent.
Reason (R): In Young's double slit experiment, we observe an interference pattern on the screen if both the slits are illuminated by two bulbs of same power.
Answer
  1. A is true but R is false.
Explanation:

lfboth the slits are illuminated by two bulbs of same power, no interference pattern will be observed on the screen. This is because waves reaching at any point on the screen do not have a constant phase difference, as phase difference from two incoherent sources changes randomly. Therefore, maxima and minima would also change their positions randomly and in quick succession. This will result in general illumination of the screen.
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Question 161 Mark
For question 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 (A): In Young's double slit experiment, the fringes become indistinct ifone of the slits is covered with cellophane paper.
Reason (R): The cellophane paper decrease the wavelength of light.
Answer
  1. A is true but R is false.
Explanation:

When one of slits is covered with cellophane paper, the intensity of light emerging from the slit is decreased (because this medium is translucent). Now the two interfering beam have different intensities or amplitudes. Hence, intensity at minima will not be zero and fringes will become indistinct.
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Question 171 Mark
For question 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 (A): Light added to light can produce darkness.
Reason (R): The destructive interference of two coherent light sources may give dark fringe.
Answer
  1. Both A and R are true and R is the correct explanation of A.
Explanation:

When light waves from two coherent sources superimpose at any particular point, crest of one wave falls on trough of the other and trough falls on the crest, the amplitude of the resultant wave is zero. Hence resultant intensity is zero. This is the phenomenon of destructive interference. Thus destructive interference produces darkness.
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Question 181 Mark
For question 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 (A): Young's double slit experiment can be performed using a source of white light.
Reason (R): The wavelength of red light is less than the wavelength of other colours in white light.
Answer
  1. A is true but R is false.
Explanation:

When source in Young's double slit experiment is of white light, the central fringe is white as all colours meet there in phase.
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Question 191 Mark
For question 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 (A): One of the condition for interference is that the two source should be very narrow.
Reason (R): One broad source is equal to large number of narrow sources.
Answer
  1. Both A and R are true and R is the correct explanation of A.
Explanation:

As a broad source is equivalent to a large number of narrow sources lying side by side. Each set of these sources will produce an interference pattern of its own which will overlap on another to such an extent that all traces of a fringe system is lost and results in general illumination. Because of this reason, for interference a narrow slit should be used.
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Question 201 Mark
For question 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 (A): For best contrast between maxima and minima in the interference pattern of Young's double slit experiment, the intensity of light emerging out of the two slits should be equal.
Reason (R): The intensity of interference pattern is proportional to square of amplitude.
Answer
  1. Both A and R are true but R is NOT the correct explanation of A.
Explanation:

When intensity of light emerging from two slits is equal, the intensity at minima,

$\text{I}_\text{min}=\Big(\sqrt{\text{I}_\text{a}}-\sqrt{\text{I}_\text{b}}\Big)^2=0$, or absolute dark.
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