Assertion & Reason Type Questions: Physical World

For following 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.
Question 1:
Assertion (A): Energy is a scalar quantity. 
Reason (R): All conserved quantities are scalars.
(a) Both A and R are true and R is the correct explanation of A
(b) 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

Ans: Option (c)
Explanation:
(i) Assertion: Energy is a scalar quantity - it has magnitude only and no direction. Examples include kinetic energy and potential energy.
(ii) Reason: The statement "All conserved quantities are scalars" is false. Some conserved quantities, such as total linear momentum and total angular momentum, are vectors.
(iii) Justification: Since the assertion is true but the reason is false, the correct code is (c).

Question 2:
Assertion (A): Classical Physics mainly includes subjects like Mechanics, Electrodynamics, Optics and Thermodynamics. 
Reason (R): The macroscopic domain includes phenomena at the laboratory, terrestrial and astronomical scales.
(a) Both A and R are true and R is the correct explanation of A
(b) 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

Ans: Option (a)
Explanation:
(i) Assertion: Classical physics does indeed consist largely of mechanics, electrodynamics, optics and thermodynamics - these describe macroscopic systems and their behaviour.
(ii) Reason: The macroscopic domain covers phenomena observable at laboratory, everyday (terrestrial) and astronomical scales, which is exactly the realm of the listed subjects.
(iii) Justification: Because the listed subjects operate in the macroscopic domain described by the reason, the reason correctly explains the assertion; hence (a) is correct.

Question 3:
Assertion (A): Wireless communication technology followed the discovery of the basic laws of electricity and magnetism. 
Reason (R): Technology gives rise to new physics.
(a) Both A and R are true and R is the correct explanation of A
(b) 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

Ans: Option (b)
Explanation:
(i) Assertion: Historically, the basic laws of electricity and magnetism were discovered before practical wireless communication was developed; the understanding of electromagnetic waves enabled wireless technology.
(ii) Reason: It is true that technological developments sometimes lead to new areas of physics or new experiments, so the reason is a true general statement.
(iii) Justification: Although both statements are true, the reason (technology giving rise to new physics) does not explain why wireless communication followed the discoveries in electricity and magnetism. Therefore the correct code is (b).

Question 4:
Assertion: Units of Rydberg constant R are m^-1.
Reason: It follows from Bohr's formula , where the symbols have their usual meaning.
(a) If both the assertion and the reason are true and the reason is a correct explanation of the assertion
(b) If both the assertion and reason are true but the reason is not a correct explanation of the assertion
(c) If the assertion is true but the reason is false
(d) If both the assertion and reason are false

Ans: Option (a)
Explanation:
(i) Assertion: The Rydberg constant R has units of reciprocal length, i.e. m^-1, which is correct.
(ii) Reason: From Bohr's formula for spectral lines, 1/λ = R_H(1/n1² - 1/n2²), R_H must have the same dimension as 1/λ, namely [L^-1].
(iii) Justification: Since the reason correctly explains why the Rydberg constant has units of m^-1, both statements are true and the reason is the correct explanation; hence (a).

Question 5:
Assertion: Parallax method cannot be used for measuring distances of stars more than 100 light years away. 
Reason: Because parallax angle reduces so much that it cannot be measured accurately.
(a) If both the assertion and the reason are true and the reason is a correct explanation of the assertion
(b) If both the assertion and reason are true but the reason is not a correct explanation of the assertion
(c) If the assertion is true but the reason is false
(d) If both the assertion and reason are false

Ans: Option (a)
Explanation:
(i) Assertion: Parallax becomes impractical for very distant stars (around or beyond ~100 light years) using simple ground-based measurements - this is a conventional observational limit because parallax angles become extremely small.
(ii) Reason: The parallax angle θ is inversely proportional to the distance r (θ ≈ baseline/r). As r increases, θ becomes very small and hard to measure accurately with typical instruments.
(iii) Justification: Because the diminishing parallax angle is the cause of the impracticality, both statements are true and the reason correctly explains the assertion; therefore (a).

Question 6:
Assertion: Number of significant figures in 0.005 is one and that in 0.500 is three.
Reason: This is because zeros are not significant.
(a) If both the assertion and the reason are true and the reason is a correct explanation of the assertion
(b) If both the assertion and reason are true but the reason is not a correct explanation of the assertion
(c) If the assertion is true but the reason is false
(d) If both the assertion and reason are false

Ans: Option (c)
Explanation:
(i) Assertion: The number 0.005 has one significant figure (the digit 5). The number 0.500 has three significant figures (5 and the two trailing zeros after the decimal). This assertion is correct.
(ii) Reason: The blanket statement "zeros are not significant" is false. Leading zeros (those before the first non-zero digit) are not significant, but trailing zeros to the right of a decimal point are significant.
(iii) Justification: Since the assertion is true but the reason is false, the correct code is (c).

Question 7: 
Assertion: Out of three measurements l = 0.7 m; l = 0.70 m and l = 0.700 m, the last one is most accurate.
Reason: In every measurement, only the last significant digit is not accurately known.
(a) If both the assertion and the reason are true and the reason is a correct explanation of the assertion
(b) If both the assertion and reason are true but the reason is not a correct explanation of the assertion
(c) If the assertion is true but the reason is false
(d) If both the assertion and reason are false

Ans: Option (b)
Explanation:
(i) Assertion: The measurement 0.700 m indicates greater precision (more significant figures) than 0.70 m or 0.7 m, so it is the most accurate among the three.
(ii) Reason: It is true that in any measurement the last significant digit carries the greatest uncertainty; this is a general principle of measurement.
(iii) Justification: Both statements are true, but the reason is a general remark about uncertainty and does not by itself explain why 0.700 m is more accurate than 0.70 m or 0.7 m. A clearer demonstration is to compare fractional uncertainties: if the uncertainty equals half the unit of the last digit, fractional errors fall with more significant figures (for example, ≈7.1%, ≈0.71% and ≈0.071% respectively). Thus both statements are true but the reason is not the specific explanation required, so (b) is correct.

Question 8: 
Assertion: Mass, length and time are fundamental physical quantities.
Reason: They are independent of each other.
(a) If both the assertion and the reason are true and the reason is a correct explanation of the assertion
(b) If both the assertion and reason are true but the reason is not a correct explanation of the assertion
(c) If the assertion is true but the reason is false
(d) If both the assertion and reason are false

Ans: Option (a)
Explanation:
(i) Assertion: Mass, length and time are called fundamental (base) physical quantities because other quantities can be derived from them.
(ii) Reason: They are independent in the sense that none of these three can be expressed in terms of the others; hence they serve as base quantities for the system of units.
(iii) Justification: Because their independence is precisely why they are regarded as fundamental, both statements are true and the reason correctly explains the assertion; thus (a) is correct.

Question 9:
Assertion: Density is a derived physical quantity. 
Reason: Density cannot be derived from the fundamental physical quantities.
(a) If both the assertion and the reason are true and the reason is a correct explanation of the assertion
(b) If both the assertion and reason are true but the reason is not a correct explanation of the assertion
(c) If the assertion is true but the reason is false
(d) If both the assertion and reason are false

Ans: Option (c)
Explanation:
(i) Assertion: Density is a derived quantity - it is calculated from mass and volume, so this is correct.
(ii) Reason: The statement "Density cannot be derived from the fundamental physical quantities" is false. Density can be expressed as mass divided by volume; volume itself is derived from the fundamental quantity length (volume ∝ length³).
(iii) Justification: Since the assertion is true but the reason is false, the correct option is (c).

Question 10:
Assertion: Nowadays a standard metre is defined in terms of the wavelength of light.
Reason: Light has no relation with length.
(a) If both the assertion and the reason are true and the reason is a correct explanation of the assertion
(b) If both the assertion and reason are true but the reason is not a correct explanation of the assertion
(c) If the assertion is true but the reason is false
(d) If both the assertion and reason are false

Ans: Option (c)
Explanation:
(i) Assertion: Historically, standards of length have been related to wavelengths of specified spectral lines of light; the statement is presented as true in the given context.
(ii) Reason: The claim "Light has no relation with length" is false. Wavelength (λ) of light is a measure of length and is related to wave speed and frequency by v = νλ.
(iii) Justification: Because the assertion is treated as true here and the reason contradicts a clear relation between light and length, the correct code is (c).