Class 12 Physics Chapter 11: Dual Nature of Radiation and Matter MCQs with answers
10 multiple-choice 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.
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Answer: (c) [ML²T⁻¹]
From E = hν, h = E/ν has the dimensions [ML²T⁻²]/[T⁻¹] = [ML²T⁻¹]. [ML²T⁻²] is energy (the work function); [MLT⁻¹] is linear momentum; [ML²T⁻³A⁻¹] is potential (the stopping potential).
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Answer: (b) the saturation current becomes three times as large and the stopping potential is unchanged
At a given frequency the saturation current is proportional to intensity, since the number of electrons emitted per second is; the stopping potential depends on the frequency and the emitter, not on intensity.
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Answer: (d) the stopping potential is larger in magnitude and the saturation current is the same
At the same intensity, curves for different frequencies reach the same saturation current, but the stopping potential is more negative for the higher frequency: the greater the frequency, the greater Kmax and hence V₀.
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Answer: (a) within about 10⁻⁹ s or less
Above the threshold frequency emission starts without apparent time lag even for very dim radiation, in a time of the order of 10⁻⁹ s or less. A delay that grows as the intensity falls is what the wave picture predicts, and is not observed.
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Answer: (b) are none at all
Emission needs hν > φ₀, that is ν > ν₀. Below ν₀ no electron is emitted, however intense the radiation or however long it falls, because each electron absorbs a single quantum, and one quantum is too small.
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Answer: (d) field emission
Energy supplied by a very strong electric field, of the order of 10⁸ V m⁻¹, gives field emission. Thermionic emission needs heating; photoelectric emission needs light of suitable frequency.
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Answer: (c) half as large
λ = h/p, so λ is inversely proportional to p: doubling p halves λ.
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Answer: (a) A photon is deflected by a magnetic field.
Photons are electrically neutral and are not deflected by electric or magnetic fields. The other three statements are photon properties: E = hν, speed c, and in a photon-particle collision the number of photons may not be conserved.
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Answer: (d) even the electrons emitted with the maximum kinetic energy are turned back
Emitted electrons have a range of energies. The current is zero when the retarding potential repels even the most energetic of them, so Kmax = eV₀. Electrons are still being emitted; they just do not reach the collector.
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Answer: (b) becomes positively charged
Ultraviolet light ejects negatively charged particles (electrons) from zinc, so an initially uncharged plate is left positively charged, as Hallwachs observed.