Important Questions for CBSE Class 12 Physics Electromagnetic Waves

1.Displacement Current The current which comes into play m the region in which the electric field and the electric flux is changing with time. It is given by

2.Need for Displacement Current Ampere’s circuital law for conduction current during charging of a capacitor was found inconsistent. Therefore, Maxwell modified Ampere’s circuital law.

3.The displacement current produces in space due to change of electric flux linked with the surface. This reveals that, varying electric field is the source of magnetic field.

4.Maxwell’s Equations of Electromagnetic Waves Maxwell’s equations are the basic laws of electricity and magnetism. These equations give complete description of ail electromagnetic interactions.

5. Electromagnetic Waves An electromagnetic wave is a wave radiated by an accelerated or oscillatory charge in which varying magnetic field is the source of electric field and varying electric field is the source of magnetic field. Thus two fields becomes source of each other and the wave propagates in a direction perpendicular to both the fields.

6. Electromagnetic waves are transverse in nature, i.e. electric and magnetic fields are perpendicular to each other and to the direction of wave propagation. Electromagnetic waves are not. deflected by electric and magnetic fields.

7. E (electric field) and B (magnetic field) in electromagnetic waves are in same phase.

8. Speed of electromagnetic wave

9. The energy in electromagnetic wave is divided on average equally between electric and magnetic fields.

10. Energy associated with an electromagnetic wave is

11. Linear momentum delivered to the surface p= U/c

where, U = total energy transmitted by electromagnetic waves and c = speed of electromagnetic wave

12. Electromagnetic Spectrum The systematic sequential distribution of electromagnetic waves in ascending or descending order of frequency or wavelength is known as electromagnetic spectrum. The range varies from 10^-12 m, to 10⁴ m, i.e. from y-rays to radio waves.

13. Elementary facts about the uses of electromagnetic waves

Radio waves

In radio and TV communication.
In astronomical field.

Microwaves

In RADAR communication.
In analysis of molecular and atomic structure.
For cooking purpose.

Infrared waves

In knowing molecular structure.
In remote control of TV VCR, etc. Ultraviolet rays

Ultraviolet rays

Used in burglar alarm.
To kill germs in minerals.

X-rays

In medical diagnosis as they pass through the muscles not through the bones.
In detecting faults, cracks, etc., in metal products,

14. The optical effect is produced by electric field vector of the electromagnetic waves.

Previous Years’ Examination Questions

1 Mark Questions

1.To which part of the electromagnetic spectrum does a wave of frequency 5 x 10¹⁹ Hz belong? [All India 2014]

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2.To which part of the electromagnetic spectrum does a wave of frequency 3 x 10¹³ Hz belong? [All India 2014]
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3.Write the condition under which an electron will move undeflected in the presence of crossed electric and magnetic fields. [All India 2014c]
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4.Welders wear special goggles or face masks with glass windows to protect their eyes from electromagnetic radiation. Name the radiations and write the range of their frequency. [All India 2013]
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5.What are the directions of electric and magnetic field vectors relative to each other and relative to the direction of propagation of electromagnetic waves? [All India 2012]

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6.Name the electromagnetic waves which
(i) maintain the earth’s warmth and
(ii) are used in aircraft navigation. [Foreign 2012]
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7.A plane electromagnetic wave travels in vacuum along Z- direction. What can you say about the direction of electric and magnetic field vectors? [Delhi 2011]
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8.Write two uses of microwaves. [Foreign 2011]
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9. Write two uses of infrared rays. [Foreign 2011]
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10.Write two uses of X-rays. [Foreign 2011]
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11.What is the frequency of electromagnetic waves produced by oscillating charge of frequency V=10⁵Hz? [Delhi 2011C]
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12.How are infrared waves produced? What is the range of their wavelength? [Delhi 2010c]
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13.Which of the following has the shortest wavelength? Microwaves, ultraviolet rays, X-rays. [All India 2010]
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14. Arrange the following in descending order of wavelength. X-rays, radiowaves, blue light, infrared light. [All India 2010]
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15. Which part of electromagnetic spectrum has largest penetrating power? [Delhi 2010]
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16. Which part of electromagnetic spectrum is absorbed from sunlight by ozone layer? [Delhi 2010]
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17. Which part of electromagnetic spectrum is used in RADAR systems? [Delhi 2010]
Ans Micro Waves

18.Name the electromagnetic radiation used to destroy cancer cells and write its frequency range. [Foreign 2010]
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19.In what way, are the directions of the electric and magnetic field vectors representing an electromagnetic wave related to each other? [Delhi 2010c]
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20.Express the velocity of propagation of an electromagnetic waves in terms of the peak value of the electric and magnetic fields. [All India 2010C]
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21.Name the electromagnetic waves used for studying crystal structure of solids. What is its frequency range? [All India 2009]
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22.Write the following radiations in ascending order in respect of their frequencies X-rays, microwaves, ultraviolet rays and radiowaves. [Delhi 2009]
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23.Name the electromagnetic radiation to which waves of wavelength in the range of 10^-2 m belongs. Give one use of this part of electromagnetic spectrum. [Delhi 2009, 2008]
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24.Name the electromagnetic radiation which can be produced by a klystron or a magnetron valve. [Foreign 2009]
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25.Why is ozone layer on top of the stratosphere crucial for human survival? [All India 2009C]
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26.State the reason why microwaves are best suited for long distance transmission of signals. [Foreign 2008]
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27.Special devices like the klystron valve or the magnetron valve, are used for production of electromagnetic waves. Name these waves and also write one of their applications. [Delhi 2008C]

2 Marks Questions

28.(a) How does oscillating charge produce electromagnetic waves?
(b) Sketch a schematic diagram depicting oscillating electric and magnetic fields of an EM wave propagating along positive Z-direction. [Foreign 2014]

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29.(i) How are electromagnetic waves produced?
(ii) How do you convince yourself that electromagnetic waves carry energy and momentum? [Delhi 2013c]
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30.(i) Arrange the following electromagnetic waves in the descending order of their wavelengths.

Microwaves
Infrared rays
Ultraviolet radiation
y-rays

(ii) Write one use each of any two of them. [Delhi 2013C]
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31.(i) An electromagnetic wave is travelling in a medium, with a velocity v = vi. Draw a sketch showing the propagation of the electromagnetic wave, indicating the direction of the oscillating electric and magnetic fields.
(ii) How are the magnitudes of the electric and magnetic fields related to velocity of the electromagnetic wave? [HOTS; Delhi 2013; All India 2008C]
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32.A capacitor of capacitance C is being charged by connecting it across a DC source along with an ammeter. Will the ammeter show a momentary deflection during the process of charging? If so, how would you explain this momentary deflection and the resulting continuty of current in the circuit? Write the expression for the current inside the capacitor. [All India 2012]
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33.When an ideal capacitor is charged by a DC battery, no current flows. However, when an AC source is used, the current flows continuously. How does one explain this, based on the concept of displacement current? [Delhi 2012]
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34.Explain briefly how electromagnetic waves are produced by an oscillating charge? How is the frequency of the electromagnetic waves produced related to that of the oscillating charge? [Foreign 2012]
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35.Name the constituent radiation of electromagnetic spectrum which is used for

aircraft navigation
studying the crystal structure Write the frequency range for each. [Delhi 2011C]

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36.Draw a sketch of a plane electromagnetic wave propagating along the Z-direction. Depict clearly the directions of electric and magnetic fields varying sinusoidally with Z. [All India 2011]

37.Arrange the following electromagnetic radiations in ascending order of their frequencies.

Microwaves
Radio waves
X-rays
y-rays

Write two uses of any one of these. [Delhi 2010]
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38.How are X-rays produced? Write their two important uses. [Foreign 2010;Delhi 2010]
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39.How are infrared rays produced? Write their two important uses. [Foreign 2010; All India 2010 C]
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40.How are microwaves produced? Write their two important uses. [Foreign 2010]

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41.Answer the following questions.

Optical and radio telescopes are built on the ground while X-ray astronomy is possible only from satellites orbiting the earth. Why?
The small ozone layer on top of the stratosphere is crucial for human survival. Why? [All India 2009]

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42.How does a charge, q oscillating at certain frequency produce electromagnetic waves? Sketch a schematic diagram depicting electric and magnetic fields for an electromagnetic wave propagating along the Z- direction. [Delhi 2009]
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43.Name the electromagnetic radiations having the wavelength range from 1 mm to 700 nm. Give its two important applications. [Foreign 2009]
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44.Name the electromagnetic radiation having the wavelength range from 10^-1 m to 10~³ Give its two important applications. [Foreign 2009]
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45.Name the electromagnetic radiations having the wavelength range from 1 nm to 10~³ Give its two important applications. [Foreign 2009]
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46.Identify the different types of electromagnetic radiations, which are used

to kill germs,
for physical therapy. [All India 2009c]

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47.In an electromagnetic wave, the oscillating electric field having a frequency of 3 x 10¹⁰ Hz and an amplitude of 30 V/m propagates in the positive X-direction.

What is the wavelength of the electromagnetic wave?
Write down the expression to represent the corresponding magnetic field. [Foreign 2008]

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48.The following table gives the wavelength range of some constituents of the electromagnetic spectrum.

S. No.	Wavelength range
1.	1 nm to 700 nm
2.	400 nm to 1 nm
3.	1 nm to10^-3 nm
4.	<10“³ nm

Select the wavelength range and name of the electromagnetic waves that are

widely used in the remote switches and household electronic devices.
produced in nuclear reactions. [Delhi 2008C]

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3 Marks Questions

49.Answer the following questions.

Name the waves which are produced during radioactive decay of a nucleus. Write their frequency range.
Welders wear special glass goggles while working. Why? Explain.
Why are infrared waves often called as heat waves? Give their one application. [Delhi 2014]

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50.Answer the following questions
(i) Name the EM waves which s suitable for radar systems used in aircraft navigation. Write the range of frequency of these waves.
(ii) If the earth did not have atmosphere, would its average surface temperature be higher or lower than what it is now? Explain.
(iii) An EM wave exerts pressure on the surface on which it is incident. Justify. [Delhi 2014]
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51.Answer the following questions.

Name the EM waves which are used for the treatment of certain forms of cancer. Write their frequency range.
Thin ozone layer on top of stratosphere is crucial for human survival. Why?
Why is the amount of the momentum transferred by the EM waves incident on the surface so small? [Delhi 2014]

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52.Answer the following questions:

Show, by giving a simple example, how EM waves carry energy and momentum.
How are microwaves produced? Why is it necessary in microwaves ovens to select the frequency of microwaves to match the resonant frequency of water molecules?
Write two important uses of infrared waves. [Delhi 2014 c]

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53.(i) Describe briefly how electromagnetic waves are produced by oscillating charges? (ii) Give one use of each of the following.

Microwaves
Ultraviolet rays
Infrared rays
y-rays [HOTS; All India 2011c]

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54.How are electromagnetic waves produced by oscillating charges? Why is it not possible to produce electromagnetic waves in the visible region with modern electronic circuits in the laboratory? What is the method of production of X-rays? [Delhi 2009C]
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