ELECTROMAGNETIC WAVES VERY SHORT ANSWER QUESTIONS Q-1. Light of uniform intensity shines perpendicularly on a totally absorbing surface, fully illuminating the surface. If the area of the surface is decreased, do (a) the radiation pressure and (b) the radiation force on the surface increase, decrease, or stay the same? Q-2. What is the frequency range of speech or music? Q-3. Why ultraviolet rays are used in some medical applications and also in sterilization processes? Q-4. Which physical quantity is the same for X-rays of wavelength 10-10 m, red light of wavelength 6800 Ǻ and radiowaves of wavelength 500 m? Q-5. What is the approximate wavelength of X-rays? Q-6. Which part of the electromagnetic spectrum does the wavelength 10-10 m corresponds to? Q-7. Q-8. Q-9. Arrange the following electromagnetic radiations in the ascending order of their wavelength. Give one medical use of UV rays. Give one use of infrared rays. Q-10. The wavelength of electromagnetic radiation is doubled. What will happen to the energy of the photon? Q-11. In electromagnetic waves, the infrared region lies between the radiowave and microwave regions. Is this statement correct? Q-12. What is the relation between amplitudes of electric and magnetic fields in free space for an electromagnetic wave? Q-13. Q-14. Arrange the following radiations in the descending order of wavelength : γ-rays, infrared rays, red light. Which of the following has the lowest frequency? Microwaves, UV rays and X-rays. Q-15. What is the nature of the waves used in Radar? What is their wavelength range? Q-16. Which of the following has the shortest wave-length? Q-17. The charging current for a capacitor is 0.25 A. What is the displacement current across its plates? 1
Q-18. Which of the following belong to electromagnetic spectrum? rays, β rays, γ rays, cathode rays, X-rays, ultraviolet rays, microwaves, ultrasonic waves, radiowaves, infrared rays. Arrange these in order of increasing frequency. Q-19. What is the principle of production of electromagnetic waves? Q-20. What is the ratio of velocities of light rays of wavelengths 4000 Ǻ and 8000 Ǻ in vacuum? Q-21. Arrange the following radiations in the descending order of wavelengths : γ rays, infrared rays, red light, yellow light, radiowaves. Q-22. Name the electromagnetic radiations used for viewing objects through haze and fog. Q-23. What is electromagnetic spectrum? Q-24. Both radiowaves and gamma rays are transverse in nature, electromagnetic in character and have the same speed in vacuum. In what respects are they different? Q-25. Rewrite the following in descending order of wavelength : Infrared rays, radiowaves, γ rays, microwaves. Q-26. Write the following radiations in a descending order of frequencies. Red light, X-rays, microwaves, radiowaves. Q-27. What is the equation for the speed of electromagnetic waves in free space? Q-28. Which part of the electromagnetic spectrum has the largest penetrating power? Q-29. What is the appropriate frequency range of the visible part of the electromagnetic spectrum? Q-30. What is the approximate wavelength range of visible spectrum? Q-31. What is the ratio of speeds of infrared rays and ultraviolet rays in vacuum? Q-32. Arrange the following radiations in an ascending order in respect of their frequencies. X-rays, microwaves, UV rays and radiowaves. Q-33. Name the scientist, who first (i) predicted the existence of electromagnetic waves. (ii) experimentally demonstrated the existence of electromagnetic waves. 2
Q-34. What is the name given to that part of electromagnetic spectrum which is used for taking photographs of earth under foggy condition from great heights? Q-35. Microwaves are used in Radar. Why? Q-36. What is the name given to that part of electromagnetic spectrum which is used in RADAR? Q-37. Write one application of microwaves. Q-38. The charging current for a capacitor is 0.2 A. What is the displacement current? Q-39. What is the ratio of velocity of light rays of wave-lengths 4000 Ǻ and 8000 Ǻ in vacuum? Q-40. Q-41. Write an expression for the speed of electromagnetic waves in free space. Which physical quantity, if any, has the same value for waves belonging to the different parts of the electromagnetic spectrum? Q-42. Name the electromagnetic waves that have frequencies greater than those of ultraviolet light but less than those of gamma rays. Q-43. What is the cause of conduction current? Q-44. Name the electromagnetic radiation to which the following wavelengths belong : (a) 10-2 m (b) 1 Ǻ. Q-45. What are radio waves? SHORT ANSWER QUESTIONS Q-46. The magnetic field of an electromagnetic wave oscillates parallel to a y-axis and given by B y = B 0y sin(kz-ωt). (a) In what direction does the wave travel and (b) parallel to which axis does the associated electric field oscillate? Q-47. What is the name associated with the following equations? i.. = ii.. = 0 iii.. = vi.. =. + 3
Q-48. In the previous question, which equations contain sources of and and which do not? What do the equations reduce to in a source-free region? Q-49. What is the contribution of the Green house effect towards the surface temperature of the earth? Q-50. Why Light waves can travel in vacuum whereas sound waves cannot? Q-51. Electromagnetic waves in a cavity with conducting walls can exist only in certain modes i.e., they cannot exist, for example, with any arbitrary wavelength, Suggest a simple reason why this should be so. Q-52. If you find closed loops of B in a region in space, does it necessarily mean that actual charges are flowing across the area bounded by the loops? Q-53. Magnetic field lines can never emanate from a point nor end on a point. Yet the field lines outside a bar magnet do seem to start from the North pole and end on the South pole. Does the second fact contradict the first? Explain. Q-54. A closed loop of is produced by a changing electric field. Does it necessarily mean that and are non-zero at all points on the loop and in the area enclosed by the loop? Q-55. Give a simple plausibility argument to suggest that an accelerated charge must emit electromagnetic radiation. Q-56. Why is it that induced electric fields due to changing magnetic flux are more readily observable than the induced magnetic fields due to changing electric fields? Q-57. Q-58. Write down Maxwell s equations for steady electric field. Compare and contrast radiowaves and gamma rays. Q-59. What does an electromagnetic wave consist of? On what factors does its velocity in vacuum depend? Q-60. Q-61. Q-62. What oscillates in electromagnetic wave? Give two examples of electromagnetic waves. Write one property and one use each of infrared rays, ultraviolet rays and radiowaves. Name any six electromagnetic waves in increasing order of their frequencies. 4
Q-63. Name the part of electromagnetic spectrum to which waves of wavelength (i) 1 Ǻ and (ii) 10-2 m belong. Using the relation λt=(0.29 cm) K, obtain the characteristic Kelvin temperature corresponding to these two wavelengths. Q-64. Electromagnetic waves with wavelength (i) λ 1 are used to treat muscular strain (iii) λ 3 are used to detect fracture in bones (ii) λ 2 are used by a FM radio station for broadcasting (iv) λ 4 are absorbed by the ozone layer of the atmosphere. Identify and name the part of the electromagnetic spectrum to which these radiations belong. Arrange these wave-lengths in decreasing order of magnitude. Q-65. A plane electromagnetic wave travels in vacuum along the y-direction. Write the (i) ratio of magnitudes and (ii) the directions of its electric and magnetic field vectors. Q-66. Identify the part of the electromagnetic spectrum which is (i) suitable for radar system used in aircraft navigation (ii) adjacent to the low frequency end of the electromagnetic spectrum (iii) produced in nuclear reaction (iv) produced by bombarding a metal target by high speed electrons. Q-67. Find the wavelength of electromagnetic waves of frequency 5 10 19 Hz in free space. Give its two applications. Q-68. Q-69. Q-70. Find the wavelength of electromagnetic waves of frequency 4 10 9 Hz in free space. Give its two applications. Find the wavelength of electromagnetic waves of frequency 6 10 12 Hz in free space. Give its two applications. Which constituent radiation of the electromagnetic spectrum is used (i) in radar (ii) to photograph internal parts of a human body, and (iii) for taking photographs of the sky during night and foggy conditions? Give one reason for your answer in each case. Q-71. Give two uses each of the following : (i) Gamma rays (iii) Ultraviolet rays (ii) Infrared rays (iv) Microwaves Q-72. A variable-frequency ac source is connected to a capacitor. Will the displacement current increases or decrease with increase in frequency? 5
Q-73. What is Maxwell s displacement current? Q-74. Q-75. Q-76. Q-77. Q-78. Q-79. What are microwaves? Give their any one use. State two properties of electromagnetic waves. Give two applications of ultraviolet radiations. Give two applications of infrared radiations. Write two applications each of (i) microwaves, (ii) infrared waves and (iii) radio waves. Name the radiations of electromagnetic spectrum which are used in (i) warfare to look through fog. (ii) radar and geostationary satellites. (iii) studying the structure and properties of atoms and molecules. Q-80. Why the small ozone layer on top of the stratosphere is crucial for human survival? Q-81. Write the order of frequency range and one use of each of the following electromagnetic radiations. (i) Microwaves (ii) Ultra-violet rays (iii) Gamma rays Q-82. Q-83. Give the frequency range and source of production of X-rays. Also write two uses of X-rays. Give the frequency range and source of production of gamma rays. Also write its any two uses. SHORT ANSWER QUESTIONS Q-84. A parallel plate capacitor is initially charged. Due to a radioactive source nearby, the medium between the plates gets slightly conducting and the plates begins to lose charge. What is the magnitude and direction of displacement current? What is the magnetic field between the plates? Q-85. A parallel plate capacitor is initially charged. Due to a nearby radioactive source, the medium between the plates gets slightly conducting and the plates lose charges. Comment on the (i) magnitude of displacement current (ii) direction of displacement current (iii) magnetic field between the plates. Q-86. Consider a plane wavefront of electromagnetic fields travelling with a speed c in the right (say + z) direction ; it is given that B and E are transverse to each other and uniform throughout the left of the wavefront and zero on the right of the wavefront. [This is a contrived, but not incorrect, configuration 6
chosen for simplicity. In the usual monochromatic plane wave, E and B vary sinusoidally in space and time]. (a) Use faraday s law to show that E=cB. (b) Use Ampere s law (with displacement current included) to show that c = Q-87. Name the constituent radiation of electromagnetic spectrum which (a) is used in satellite communication. (b) is used for studying crystal structure. (c) is similar to the radiations emitted during decay of radioactive nuclei. (d) has its wavelength range between 390 nm and 770 nm. (e) is absorbed from sunlight by ozone layer. (f) produces intense heating effect. Business Q-88. Name the following constituent radiations of electromagnetic spectrum which (i) produce intense heating effect (ii) is absorbed by the ozone layer in the atmosphere (iii) is used for studying crystal structure. Write one more application for each of these radiation CONCEPTUAL PROBLEMS Q-89. (a) Let us take it as given that Maxwell s equations predict the existence of electromagnetic waves in vacuum with a constant speed c = = 3 10 8 ms -1, Do you find something strange in the appearance of a constant in the theory that has dimensions of speed? Explain. (b) The hypothesis of ether (a medium permeating all of space, and endowed with very special properties) is a natural idea based on Maxwell s equations (through we know now that the hypothesis is incorrect). Explain why the ether is such a natural idea? (c) Are Maxwell s equations modified by Einstein s special relativity? Are they valid in the domain of quantum theory? Q-90. Check the dimensional consistency of Maxwell s equations and the Lorentz force law. 7
Q-91. The four Maxwell s equations and the Lorentz force law (which together constitute the foundation of all of classical electromagnetism) are listed below : i.. = ii.. = 0 iii.. = vi.. = +.. Lorentz force law : = q ( + ). (a) Give the name (s) associated with some of the four equations above. (b) Which equations above contain sources of and and which do not? What do the equations reduce to in a source-free region? (c) Write down Maxwell s equations for steady (i.e., time independent) electric and magnetic fields. (d) If magnetic monopoles existed, which of the equations would be modified? Suggest how they might to modified? (e) Which of the four equations shows that magnetic field lines cannot start from a point nor end at a point? (f) Which of the four equations shows that electrostatic field lines cannot form closed loops? (g) The equations listed above refer to integrals of E and B over loops /surfaces. Can we write down equations for E and B each point in space? (h) Are the equations listed above true for different types of media : dielectrics, conductors, plasmas etc.? (i) Are the equations true for arbitrarily high and low values of E, B, q, I? LONG ANSWER QUESTIONS Q-92. Q-93. Q-94. State six characteristics of electromagnetic waves. Describe an experiment to demonstrate the transverse nature of light. Draw a labelled diagram of Hertz s experimental setup to produce electromagnetic waves. Explain the generation of electromagnetic waves using this setup. 8
Q-95. Draw a labelled diagram of the experimental set-up used by Hertz to produce electromagnetic waves. Explain how the electromagnetic waves are detected. Q-96. Describe the Hertz s experiment for the production of electromagnetic waves. 9