PHOTOELECTRIC EFFECT VERY SHORT ANSWER QUESTIONS Q-1. What is the practical unit of work function? Q-2. Is it necessary that each and every photon of the incident light would emit a photoelectron? Q-3. How many photons are required to emit one photoelectron? Q-4. What is the effect of decrease in intensity on stopping potential? Q-5. What is the effect of increase in intensity on photoelectric current? Q-6. Q-7. Name one factor on which threshold frequency depends. Name one factor on which the retarding potential depends. Q-8. Complete the following statements : (i) The maximum kinetic energy of photoelectrons depends on and.but not on. (ii) The saturated photoelectric current increases in proportion to, while the stopping voltage increases linearly with the.of radiation. (iii) The threshold frequency for photoelectric emission from platinum is greater than that of zinc, which in turn is greater than that of sodium. An incident radiation of frequency v > v 0 (Zn) cause photoelectric emission from sodium but.cause photoelectric emission from platinum. Q-9. Assuming that electrons are free inside a solid, sketch graphically the distribution n(v) of electrons with speed v. Q-10. What is the effect on the velocity of the emitted photoelectrons if the wavelength of the incident light is decreased? Q-11. If the intensity of incident radiation on a metal surface is doubled, what happens to the kinetic energy of the electrons emitted? Q-12. How many electron volt make one joule? Q-13. The maximum kinetic energy of electrons emitted by a photocell is 3 ev. What is the stopping potential? 1
Q-14. Does the threshold frequency depend on intensity of light? Q-15. How does the maximum kinetic energy of electrons emitted vary with the work function of the metal? Q-16. If the intensity of incident radiation in a photocell is increased, how does the stopping potential vary? Q-17. The frequency of incident radiation is greater than threshold frequency in a photocell. How will the stopping potential vary, if frequency is increased, keeping other factors constant? Q-18. The maximum kinetic energy of electrons emitted by a photocell is 5 ev. What is the stopping potential? Q-19. Q-20. Q-21. Name a phenomenon which illustrates the particle nature of light. State Einstein s photoelectric equation in mathematical form. In a photoelectric effect experiment, the following graphs were obtained between the photoelectric current and the applied voltage. Name the characteristic of the incident radiation that was kept constant in this experiment. Q-22. What is photoelectric effect? Q-23. Define threshold wavelength for photoelectric effect. Q-24. What are photoelectrons? Q-25. On what factors does the energy carried by a quantum of light depend? Q-26. Which photon is more energetic : a red one or a yellow one? Q-27. Why are alkali metals most suited for photoelectric emission? Q-28. Define the term work function for photoelectric effect. 2
Q-29. How will the photoelectric current change on decreasing the wavelength of incident radiation for a given photosensitive material? Q-30. What is stopping potential in photoelectric effect? Q-31. Why is no electron emitted from a plastic plate, when light from a bulb falls on it? SHORT ANSWER QUESTIONS Q-32. Fig 22.19 Show data for targets of caesium, potassium, sodium, and lithium. The plots are parallel. (a) Rank the targets according to their work functions, greatest first. (b) Rank the plots according to the value of h they yield, greatest first. Q-33. The work functions of two metals A and B are 1 ev and 4 ev respectively. Which of these two would be suitable for use in a photocell where visible light is used? Q-34. Light of frequency v is made incident on a block of three different materials P, Q and R. All the materials behave differently. Guess the possible different behaviours. Q-35. For each statement below, state if it is true or false. (a) The maximum kinetic energy of photoelectrons depends on the frequency of radiation and material of the photocell. (b) If the intensity of incident radiation is doubled, the maximum kinetic energy of photoelectrons is also doubled. (c) The cut-off voltage in a photoelectric experiment has a linear dependence on the frequency of incident radiation. (d) The photon picture of electromagnetic radiation predicts nearly instantaneous photoelectric emission. Q-36. What is the frequency associated with a photon of energy 3.3 10-10 J Given : h = 6.6 10-34 J s. Q-37. Is photoelectric emission possible at all frequencies? Given reasons for your answer. 3
Q-38. All the photoelectrons are not emitted with the same energy. The energies of photoelectrons are distributed over a certain range. Why? Q-39. What is the energy associated in joule with a photon of wavelength 4000 Ǻ? Q-40. A radio transmitter operates at a frequency of 880 KHz and a power of 10 kw. Find the number of photons emitted per second. Given : h = 6.6 10-34 Js. Q-41. Every metal has a definite work function. Why do photoelectrons not come out all with the same energy if incident radiation is monochromatic? Why is there an intensity distribution of photoelectrons? Q-42. Q-43. Calculate the frequency of a photon with energy 7.5 ev. Given : h = 6.62 10-34 Js. If the frequency of incident radiation on a photocell is doubled for the same intensity, what changes will you observe in (i) kinetic energy of photoelectrons emitted (ii) photoelectric current? Q-44. Radiation of frequency 10 15 Hz is incident on two photosensitive surfaces P and Q. Following observations are made : (i) Surface P : Photoemission occurs but the photoelectrons have zero kinetic energy. (ii) Surface Q : Photoemission occurs and photoelectrons have some kinetic energy. Which of these has a higher work function? If the incident frequency is slightly reduced, what will happen to photoelectron emission in the two cases? Q-45. The maximum kinetic energy of photoelectrons emitted from a surface, when is the stopping potential (in volt) for the fastest photoelectrons? Q-46. Two metals X and Y, when illuminated with appropriate radiation, emit photoelectrons. The work function of X is higher than that of Y. Which metal will have higher value of threshold frequency and why? Q-47. A source of light is placed at a distance of 50 cm from a photocell and the cut-off potential is found to be V 0. If the distance between the light source and photocell is made 25 cm, what will be the new cut-off potential? Justify your answer. 4
Q-48. Name the device that converts changes in intensity of illumination into changes in electric current. Give three applications of this device. Q-49. Q-50. What are photons? State their properties. Two beams, one of red light and the other of blue light, of the same intensity are incident on a metallic surface to emit photoelectrons. Which one of the two beams emits electrons of greater kinetic energy? Q-51. Two metals A and B have work functions 4 ev and 10 ev respectively. Which metal has higher threshold wavelength? Q-52. Red light, however bright it is, cannot produce the emission of electrons from a clean zinc surface. But even weak ultraviolet radiation can do so. Why? Q-53. If the frequency of incident radiations is equal to the threshold frequency, what will be the value of stopping potential? Q-54. Ultraviolet light is incident on two photosensitive materials having work functions W 1 and W 2 (W 1 > W 2 ). In which case will the kinetic energy of the emitted electrons be greater? Why? Q-55. A metal emits electrons if green light falls on it but this is not true with yellow light. Will it emit electrons with red light? Will it emit electrons with blue light? Q-56. Does the stopping potential in photoelectric emission depend upon (i) the intensity of the incident radiation in a photocell? (ii) the frequency of the incident radiation? Q-57. Plot a graph showing the variation of photoelectric current with anode potential for two light beams of same wavelength but different intensity. Q-58. Q-59. Q-60. Write two uses of photoelectric cell. Which photon is more energetic : blue one or red one? Give reason. Define the term threshold frequency and work function in relation to photoelectric effect. 5
Q-61. Sketch a graph between frequency of incident radiations and stopping potential for a given photosensitive material. What information can be obtained from the value of the intercept on the potential axis? Q-62. A source of light of frequency greater than the threshold frequency is placed at a distance of 1 m from the cathode of a photocell. The stopping potential is found to be V. If the distance of the light source from the cathode is reduced, explain giving reasons, what change will you observe in the (i) photoelectric current (ii) stopping potential does not depend upon intensity therefore the stopping potential? Q-63. Ultraviolet radiations of different frequencies v 1 and v 2 are incident on two photosensitive materials having work functions W 1 and W 2 (W 1 > W 2 ) respectively. The kinetic energy of the emitted electrons is same in both the cases. Which one of the two radiations will be of higher frequency? SHORT ANSWER QUESTIONS Q-64. Complete the statements below choosing appropriate clues from accompanying lists : (i) If the intensity of incident radiation is doubled, the number of photoelectrons emitted per second by the cathode is.but the maximum kinetic energy of electrons is.(unaltered, doubled). (ii) The wave picture of radiation does not have a natural explanation for.and.(existence of a threshold frequency, dependence of current on intensity, dependence of stopping voltage on frequency but not on intensity). (iii) The.predicts nearly instantaneous photoelectric emission unlike the which requires considerable time for electrons to absorb energy before emission (wave picture, photon picture). Q-65. Prove that the product of the slope of v-v s graph and electronic charge gives the value of Planck s constant. Q-66. Light of wavelength 5000 Ǻ falls on a sensitive plate with work function 1.90 ev. Calculate (a) the energy of the photon in ev, (b) kinetic energy of the emitted photoelectrons and (c) stopping potential. Given : h = 6.62 10-34 Js. 6
Q-67. For a photosensitive surface, threshold wave length is λ 0. Does photoemission occur, if the wavelength (λ) of the incident radiation is (a) more than λ 0 (b) less than λ 0? Justify your answer. Q-68. In a photoelectric experiment, the graph between the stopping potential V and frequency v of the incident radiations on two different metal plates P and Q are shown in figure. (i) Which of the two metal plates P and Q has greater value of work function? (ii) What does the slope of the lines depict? Q-69. For photoelectric effect in a metal, Fig 22.24 shows the plot of cut-off voltage versus frequency of incident radiation. Determine (i) the threshold frequency (ii) work function of the given metal. Q-70. Define the terms threshold frequency and stopping potential in relation to the phenomenon of photoelectric effect. How is the photoelectric current affected on increasing the(i) frequency (ii) intensity of the incident radiations and why? Q-71. Draw the graphs showing the variation of photoelectric current with anode potential of a photocell for (i) the same frequencies but different intensities I 1 > I 2 > I 3 of incident radiation. (ii) the same intensity but different frequencies v 1 > v 2 > v 3 of incident radiation. Explain why the saturation current is independent of the angle potential. Q-72. Sketch the graphs showing the variation of stopping potential V 0 with frequency v of incident radiations for two photosensitive metal A and B having threshold frequencies v 0 and v 0 respectively (v 0 > v 0 ). (i) Which of the two metal, A and B has higher work function? (ii) What information do you get from the slop of the graphs? (iii) What does the value of the intercept of graph A on the potential axis represent? Q-73. The following table gives the values of work function for a few photosensitive metals. S.No Metal Work Function (ev) 1. 2. 3. Na K Mo 1.92 2.15 4.17 If each of the metal is exposed to radiations of wavelength 300 nm, which of them will not emit photoelectrons and why? 7
Q-74. By how much would the stopping potential for a given photosensitive surface go up if the frequency of the incident radiations were to be increased from 4 10 15 Hz to 8 10 15 Hz? Given h = 6.4 10-34 J s, e = 1.6 10-19 C and c = 3 10 8 m s -1. Q-75. The given graphs show the variation of photoelectric current (I) with the applied voltage (V) for two different materials and for two different intensities of the incident radiations. Identify the pairs of curves that correspond to different materials but same intensity of incident radiations. Q-76. The given graphs show the variation of the stopping potential V 8 with the frequency (v) of the incident radiations for two different photosensitive materials M 1 and M 2 (i) What are the values of work functions for M 1 and M 2? (ii) The value of the stopping potential for M 1 and M 2 for a frequency v 3 (>V0 2 ) of the incident radiations are V 1 and V 2 respectively. Show that the slope of the lines equals V 1 V 2 V0 2 V0 1 CONCEPTUAL PROBLEMS Q-77. Threshold wavelength for a certain material is 5000 Ǻ. Will photoemission take place when this material is illuminated with light from 8.3 W UV lamp? 8
Q-78. Light of frequency 1.5 time the threshold frequency is mode incident on a photosensitive material. If intensity is doubled and frequency is halved, then what will be the effect on photoelectric current? Q-79. What is the physical significance of the statement : The work function of a metal is x ev? Q-80. A source of light is placed at a distance r from a photocell. What would be the effect on the cut-off potential if distance is doubled? Q-81. A good mirror reflects 80% of light incident on it. How would you determine that 20% photons have not been reflected or all photons are reflected but energy of each is decreased by 20%? Q-82. Will photoelectrons be emitted by a copper surface of work function 4.4 ev, when illuminated by visible light? Q-83. If the frequency of light incident on a metallic plate be doubled, will the kinetic energy of electrons be also doubled? Q-84. Will sodium show photoelectric effect with orange light of wavelength 6800 Ǻ? The work function of sodium is 2.3 ev. Q-85. An X-ray photon of wavelength λ strikes an electron at rest and suffers Compton scattering. Will the scattered photon have a wavelength greater or smaller than λ? Q-86. What happens to the wavelength of a photon after it collides with an electron? Q-87. What is the frequency of a photon of energy 3.3 10-20 J? Q-88. Two mental A and B have work functions 2 ev, 4 ev respectively. Which metal has a lower threshold wavelength for photoelectric effect? LONG ANSWER QUESTIONS Q-89. Q-90. Q-91. Q-92. Describe photoelectric effect and state the laws of photoelectric emission. Explain laws of photoelectric emission. Explain the working of a photocell. What is Photoelectric effect? State its laws and derive these laws from Einstein s Photoelectric equation. 9
Q-93. Define Photoelectric effect, work function, threshold frequency and stopping potential of a metal. State the laws of photoelectric emission. Give two applications of photoelectric current in daily life. Q-94. What is Photoelectric effect? Discuss suitable experiments to study its laws. 10