Wednesday, July 27, 2011

Question Bank on Electromagnetic Waves for Class 12 CBSE


Properties of Electromagnetic Waves

Electromagnetic waves are composed of oscillating electric and magnetic fields at right angles to each other and both are perpendicular to the direction of propagation of the wave. Electromagnetic waves differ in wavelength (or frequency).

In  an electronegative wave the electric field E(vector) and the Magnetic field B(vector) oscillate perpendicular to each other and both are perpendicular to direction of propagation of wave.

The source that produce them and methods of their detection are different, but they have the following common properties :
1. Electromagnetic waves are propagated by oscillating electric and magnetic fields oscillating at right angles to each other.
2. Electromagnetic waves travel with a constant velocity of  3 x 108 ms-1 in vacuum.
3. Electromagnetic waves are not deflected by electric or magnetic field.
4. Electromagnetic waves can show interference or diffraction.
5. Electromagnetic waves are transverse waves.
6. Electromagnetic waves may be polarized.
7. Electromagnetic waves need no medium of propagation. The energy from the sun is received by the earth through electromagnetic waves.
8. The wavelength (λ) and the frequency (v) of electromagnetic wave is related as
c = v λ = ω/k
The S.I. unit of frequency is Hertz.
1 Hertz = 1 c / s
The S.I. unit of wavelength is metre.
We however, often express wavelength in Angstrom unit [ Ã… ]
1 Ã… = 10-10 m
Also, 1 nanometer = l nm = 10-9 m

(In the chapter electromagnetic waves, only 1 mark, 2 marks or 3 marks questions will be asked in Board Exams. There is no chance for a 5 marks question. So, we've included all in a single section.)

  1. Discuss the inconsistency in Ampere's Circuital Law.

  2. What modification was made to Ampere's Circuital Law by Maxwell?

  3. What is displacement current?

  4. Conduction current and displacement current can be separately discontinuous. But their sum is continuous. Explain.

  5. Can displacement current (or a changing electric flux induce a magnetic field? Explain

  6. State the important consequences of displacement current.

  7. State the important properties of displacement current.

  8. Write Maxwell's equations of electromagnetism and state the law underlying each equation.

  9. Explain what led Maxwell to predict the existence of electromagnetic waves.

  10. How are electromagnetic waves represented mathematically?

  11. What is the dimension of (a)μ0 ε0(b)√(μ0 ε0)

  12. Discuss how an accelerating charge becomes a source of an electromagnetic wave?

  13. Describe Hertz's experiment for producing and detecting electromagnetic waves.

  14. Explain how Hertz demonstrated the various properties of electromagnetic waves?

  15. List the different landmarks in the history of electromagnetic waves.

  16. Prove the transverse nature of electromagnetic waves mathematically.

  17. Write the properties of electromagnetic waves.

  18. What is electromagnetic spectrum.

  19. Name the main parts of the electromagnetic spectrum and mention their frequency range and source of production. Also write their important properties and uses.


  1. If you find closed loops of magnetic field in a region of space, does it necessarily mean that actual charges are flowing across the area bounded by the loops? Explain.

  2. 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?

  3. A variable frequency AC source is connected to a capacitor. Will the displacement current increase or decrease with increase in frequency?

  4. Why is the quantity ε0 dφE/dt called displacement current?

  5. What oscillates in am Electromagnetic Wave?

  6. Why sound waves are not considered em waves?

  7. What is the phase relationship between the oscillating electric and magnetic fields in an electromagnetic wave?

  8. X- rays and γ rays have a region of overlapping frequencies and have similar characteristics. But why they are treated as different? (OR) How are γ rays different from X rays?

  9. Why is the ozone layer of atmosphere crucial to the existence of life on earth?

  10. Why does "RADAR" use microwaves?

  11. In a microwave oven, the food kept in a closed plastic container get s cooked without melting or burning the plastic container. Explain how?

  12. Can an electromagnetic wave be deflected by an electric or magnetic field? Justify your answer.

  13. Name the constituent radiation of electromagnetic spectrum which
    (a) is used in satellite communication
    (b)Is used for studying crystal structure
    (c)is emitted during decay of radio active nuclei
    (d) is absorbed by ozone layer Write two more uses of each

  14. What is the dimension of E/B?

  15. Show that the dimensions of are that of electric current

  16. he sunlight reaching the earth has a maximum electric field of 810 V/m. What is the maximum magnetic field associated with it?

  17. Calculate the relative permitivity of a medium of relative permeability 1.0 if the velocity of light through the medium is 2 x 108 m/s.

  18. Which are the different layers of earth's atmosphere?

(There are no limits to the Hots Questions to be framed from ANY chapter. What is important is to understand the concepts thoroughly and apply your brain)

Numerical Problems from ELECTROMAGNETIC WAVES for CBSE Class XII

  1. A parallel plate capacitor has circular plates, each of radius 5.0 cm. It is being charged so that electric field in the gap between its plates rises steadily at the rate of 1012V m-1s-1. What is the displacement current?

  2. The voltage between the plates of a parallel plate capacitor of capacitance 1.0 μF is changing at the rate of 5 V s-1. What is the displacement current in the capacitor?

  3. A parallel plate capacitor has two metal plates of size 30 cm X 15 cm and separated by 2.0 mm. The capacitor is being charged so that the charging current has a steady value of 100 mA. Calculate the rate of change of potential difference between the capacitor plates. What is the displacement current between the capacitor plates?

  4. The frequencies of radio waves in AM broadcast band range from 0.55 x 106 to 1.6 x 106 Hz. What are the longest and shortest wavelengths in this band?

  5. A plane electromagnetic wave of frequency 25 MHz travels in free space along X - direction. At a particular point in space and time, E = 6.3 j V/m. What is B at this point?

There are many other similar Numerical Problems which may be asked in CBSE Exam

Some Questions and Solutions can be downloaded from here

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