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Class 12 Physics Chapter 10: Wave Optics Assertion-reason questions with answers

2 assertion-reason questions on Wave Optics, each with its answer and the reason, written from the NCERT chapter. For the CBSE board, 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): In Young's experiment the pinholes S₁ and S₂, lit by a single pinhole S, behave as coherent sources. Reason (R): Any abrupt change of phase in the light from S appears as exactly similar changes at S₁ and S₂, so their phase difference stays fixed.
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Answer: (a) Both A and R are true, and R is the correct explanation of A.

Both statements are true, and the locking of the phases through the common source S is exactly why S₁ and S₂ are coherent.

Question 2
Assertion (A): The behaviour of light with polaroids shows that light waves are transverse. Reason (R): Interference and diffraction effects exist even for longitudinal waves such as sound in air.
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Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Both are true. Polarisation is special to transverse waves, which is why polaroids show that light is transverse. R is true but explains nothing about A; it says interference and diffraction cannot tell transverse from longitudinal waves.

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