Physics · Dual nature of radiation and matter · chapter 11 of 14

Class 12 Physics Chapter 11: Dual Nature of Radiation and Matter Assertion-reason questions with answers

2 assertion-reason questions on Dual Nature of Radiation and Matter, 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): An electron and a proton, each accelerated from rest through the same potential difference, have different de Broglie wavelengths, the electron's being the longer. Reason (R): Both gain the same kinetic energy eV, and λ = h/√(2mK) is longer for the smaller mass.
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Answer: (a) Both A and R are true, and R is the correct explanation of A.

Both carry charge of magnitude e, so each gains K = eV. With λ = h/√(2mK), the same K gives the longer wavelength to the lighter electron. R is true and explains A.

Question 2
Assertion (A): The graph of stopping potential against frequency has the same slope for every photosensitive metal. Reason (R): The threshold frequency differs from metal to metal.
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Answer: (b) Both A and R are true, but R is not the correct explanation of A.

A is true: the slope is h/e, a ratio of universal constants. R is also true, but it explains why the lines for different metals cut the frequency axis at different points, not why their slopes are equal.

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