CBSE Physics Electrons and Photons Expected Questions : Unit IX
(Derivation Type/NumericaIs-3 Marks)
1. Radiation of frequency 10 Hz is incident on two photosensitive surfaces P and Q. Following observations are recorded.
(() At surface P, no photoemission occurs.
(ii) At surface B, photoemission occurs but photoelectrons have zero kinetic energy.
Based upon Einstein’s photoelectric equation, explain the two observations.
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2. The voltage across the electrodes of a cathode gun is 500 V. Calculate (i) the energy gained by electrons,
(ii) the speed of the electrons.
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3. For photoelectric effect in sodium, the figure shows the plot of cut off voltage verses frequen¬cy of the incident radiation. Calculate :
(i) the threshold frequency.
(ii) the work function for sodium.
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4. If a photosensitive surface has threshold frequency of 4.6 X 1014 Hz, Calculate the energy of the photon in eV.
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5. Explain the functioning of a photocell. Give its two uses.
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6. What is the energy associated in joules with a photon of wavelength 4,000 A ?
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7. The total energy of an electron in the first excited state of the H-atom is — 3.4e V. What are the values of potential and kinetic energies of the electrons in this state ?
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8. The graphs between stopping potential V and frequency v of the incident radiation on two different metal plates P and Q are shown in figure.
(a) Which metal, out of P and Q has greater value of work function ?
(b) What does the slope of line depict ?
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9. The energy level of an element X are shown in the diagram. A photon of wavelength 620 nm is emitted. This corresponds to which transitions, A, B, C, D or E ?
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