CBSE Class 12 Physics Dual Nature Of Matter Frequently Asked Questions

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DUAL NATURE OF MATTER

1. When a monochromatic yellow coloured light beam is incident on a given photosensitive surface, photoelectrons are not ejected, while the same surface gives photoelectrons when exposed to green coloured monochromatic beam. What will happen if the surface is exposed to: (i) red coloured, monochromatic beam of light? Justify your answer.

2. What is meant by work function of a metal? How does the value of work function influence the kinetic energy of electrons liberated during photoelectric emission?

3. Define the terms: (i) work function, (ii) threshold frequency & (iii) stopping potential with reference of photoelectric effect.

4. The work function of lithium is 2.3 eV. What does it mean? What is the relation between the work function ‘ωo’ and threshold wavelength ‘λo’ of a metal?

5. Red light, however bright, cannot cause emission of electrons from a clean zinc surface. But, even weak ultraviolet radiations can do so. Why?

6. An electron and a proton have same kinetic energy. Which of the two has a greater wavelength? Explain.

7. Define the term threshold frequency & work function in relation to photoelectric effect.

8. An electron and a proton are moving in the same direction and possess same kinetic energy. Find the ratio of de-Broglie wavelengths associated with these particles.

 9. In the photoelectric effect experiment, the graph between the stopping potential ‘V’ and frequency ‘n’ of the incident radiation on two different metal plates P and Q are shown in the figure. (i) Which of the two metal plates, P & Q has greater value of work function? & (ii) What does the slope of the line depict?

10. What is photoelectric effect? Write Einstein’s photoelectric equation and use it to explain: (i) independence of maximum energy of emitted photoelectrons from the intensity of incident light. (ii) Existence of a threshold frequency for the emission of photoelectrons.

11. Draw the variation of maximum kinetic energy of emitted electrons with frequency of the incident radiation on a photosensitive surface. On the graph drawn, what do the following indicate: (i) slope of the graph & (ii) intercept on the energy axis.

12. Obtain Einstein’s photoelectric equation. Explain how it enables us to understand the (i) linear dependence of the maximum kinetic energy of the emitted electrons, on the

frequency of the incident radiation & (ii) existence of a threshold frequency for a given photo emitter.

13. Given below is the graph between frequency (n) of the incident light and maximum kinetic energy (E) of emitted photoelectrons. Find the values of: (i) threshold frequency and (ii) work function from the graph.

14. Sketch a graph between frequency of incident radiations and stopping potential for a given photosensitive materials. What information can be obtained from the value of

intercept on the potential axis? A source of light of frequency greater that the threshold frequency is replaced at a distance of 1 m from the cathode of a photo cell.

The stopping potential is found to be V. If the distance of the light source from the cathode is reduced, explain giving reason, what change you will observe in the (I) photoelectric current & (ii) stopping potential.

15. Explain the laws of photoelectric emission on the basis of Einstein’s photoelectric equation. Write one feature of the photoelectric effect which cannot be explained on the basis of wave theory of light.

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