Physics · Oscillations and waves · chapter 14 of 14

Class 11 Physics Chapter 14: Waves Assertion-reason questions with answers

2 assertion-reason questions on Waves, each with its answer and the reason, written from the NCERT chapter. For school exams on the CBSE pattern, JEE Main and NEET, which are set from the same NCERT chapter. Open a question's solution to check your working; then take the free test to find your next chapter. Each question gives an Assertion (A) and a Reason (R); choose how they stand to each other.

Question 1
Assertion (A): When the frequency of the vibrator driving a stretched string is doubled, the speed of the waves on the string does not change. Reason (R): The speed of a transverse wave on a stretched string depends only on the tension and the mass per unit length of the string.
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Answer: (a) Both A and R are true, and R is the correct explanation of A.

Both are true. v = √(T/μ) is set by the string, not by the source; the source fixes the frequency, and the wavelength adjusts through λ = v/ν. R is exactly why A holds.

Question 2
Assertion (A): In a stationary wave on a string, all the elements between two adjacent nodes oscillate in phase. Reason (R): The distance between two successive nodes of a stationary wave is half a wavelength.
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Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Both are true. In y = 2a sin kx cos ωt every element oscillates as cos ωt, with an amplitude 2a sin kx whose sign is the same between two adjacent nodes, so they move in phase. The node spacing λ/2 is correct but says nothing about phase.

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