Bihar Board - Class 12 Physics - Chapter 8: Electromagnetic Waves Short Answer Question
Short Answer Type Questions
1. What are electromagnetic waves?
Answer: Electromagnetic waves are waves propagating in free space (vacuum), without any material medium, caused by coupled time-varying (oscillating) electric and magnetic fields.
2. What is the nature of electromagnetic waves?
Answer: The electromagnetic waves are transverse in nature. Directions of oscillating electric field E and magnetic field B are perpendicular to each other as well as perpendicular to the direction of propagation of electromagnetic waves.
3. How does a charge oscillating at a certain frequency produce electromagnetic waves?
Answer: The oscillating charge produces an oscillating electric field in space. It produces an oscillating magnetic field, which, in turn, is a source of oscillating electric field and so on. In this manner, the oscillating electric and magnetic fields regenerate each other and as a result, an electromagnetic wave propagates through free space.
4. What are the directions of electric and magnetic field vectors relative to each other and relative to the direction of propagation of electromagnetic waves?
Answer: The directions of electric field E and magnetic field B are mutually perpendicular and simultaneously both of them are in perpendicular directions relative to the direction of propagation of electromagnetic waves.
5. What is displacement current?
Answer: Displacement current is the current set up in space due to a time changing electric field or electric flux.
Mathematically, it is given by
Displacement current, ID = 0.dEdt
6. A plane electromagnetic wave travels in vacuum along y-direction. What can you say about the direction of electric and magnetic field vectors?
Answer: When a plane electromagnetic wave travels in vacuum along y-direction, then electric field E and magnetic field vector B are along z-direction and x-direction respectively or vice versa.
7. What feature of electromagnetic waves led Maxwell to conclude that light itself is electromagnetic wave?
Answer: The fact that electromagnetic waves can travel through vacuum with a velocity equal to the velocity of light (c=3×108m/s) led Maxwell to conclude that light itself is an electromagnetic wave.
8. How are amplitudes of oscillating electric and magnetic fields related to?
Answer: The ratio of amplitudes of electric and magnetic fields is equal to the speed of light. Mathematically,
E0B0 = C
9. What is the phase relationship between oscillating electric and magnetic fields in an electromagnetic wave?
Answer: The electric field E and magnetic field B in an electromagnetic wave are exactly in same phase condition.
10. Write the following radiations in ascending order in respect of their frequencies:
X-rays, microwaves, ultraviolet rays and radio waves
Answer: The radiations arranged in ascending order in respect of their frequencies are radio waves, microwaves, ultraviolet rays and X-rays.
11. Arrange the given electromagnetic radiation in the descending order of their frequencies: Infrared rays, X-rays, ultraviolet rays and γ-rays
Answer: The radiations arranged in descending order in respect of their frequencies are γ-rays, X-rays, ultraviolet rays and infrared rays.
12. From the following, identify the electromagnetic waves having the (a) maximum, and (b) minimum frequency. Radio waves, γ-rays, visible light, microwaves, UV rays and infrared rays.
Answer: Out of various types of electromagnetic waves given here:
(a) gamma rays have maximum frequency, and
(b) radio waves have minimum frequency.
13. Arrange the following radiations in an ascending order of their wavelengths: Infrared rays, radio waves, γ-rays and microwaves .
Answer: The radiations arranged in an ascending order of wavelengths are γ-rays, infrared rays, microwaves and radio waves.
14. Arrange the following radiations in descending order of their wavelengths: X-rays, radio waves, blue light, infrared light
Answer: The radiations arranged in descending order of their wavelengths are radio waves. infrared light, blue light and X-rays.
15. Name the physical quantity which remains the same for microwaves of wavelength 1 mm and UV radiations of 1600 Å in vacuum.
Answer: Both microwaves and UV radiations travel with the same speed of 3×108 m/s in vacuum in spite of the difference in their wavelengths.
16. Write the frequency limits of the visible light portion of the electromagnetic spectrum.
Answer: Visible light region of electromagnetic spectrum extends from about 4.3 x 1014 Hz to 7.5 x 1014 Hz.
17. Name the electromagnetic radiation to which waves of wavelength in the range of 10-2 m belong. Give one use of this part of the electromagnetic spectrum.
Answer: Microwaves have wavelengths in the range of 10-2 m. These are used in RADAR systems, in microwave telecommunication and in microwave ovens.
18. Name the part of the electromagnetic spectrum of wavelength 102 m and mention its one application.
Answer: Radio waves are the electromagnetic waves of wavelength 102 m. These are used for short-band radio communication.
19. Name the electromagnetic waves that have frequencies greater than those of ultraviolet light but less than those of γ-rays.
Answer: Frequencies of X-rays are greater than those of ultraviolet light but less than those of γ-rays
20. Which part of the electromagnetic spectrum is absorbed from sunlight by the ozone layer?
Answer: Ultraviolet rays are absorbed from sunlight by the ozone layer.
21. Name the electromagnetic radiations used for studying crystalline structure of solids.
Answer: X-rays are used for studying crystalline structure of solids.
22. Which part of the electromagnetic spectrum is used for LASIK eye surgery, and why?
Answer: Ultraviolet rays are generally used for LASIK eye surgery because being of shorter wavelengths, these can be easily focussed into very narrow beams for high precision required in the surgery.
23. Which part of the electromagnetic spectrum is used in radar systems?
Answer: Microwaves are used in radar systems used in aircraft navigation.
24. Can displacement current be a source of magnetic field?
Answer: Yes, just like conduction current, the displacement current is a source of magnetic field.
25. What is the basic difference between displacement current and conduction current?
Answer: Conduction current is due to flow of current carriers (generally, conduction electrons in metallic conductors) in the circuit. Displacement current is due to time-varying electric field and set up in space even when no conductor is present. Again, conduction current flows even for steady electric fields. But displacement current does not exist in the state of a steady electric field.
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