- CHAPTER DB

ElectromagneticWaves

Chapter Analysis w.r.t, Lost 3 Year's Board ExamsThe analysis given here gives you an analytical picture of this chapter and will helpyou toidentify the concepts of the chapter that are to befocussed morefrom exam point of view.

Number of Questions asked in last 3 yearsr-----------.----------r------------,------------~-------------2015 2016 2017

Delhi All India Delhi All India .Delhi All India

Very Short An~:;;-;:-~~ - 1 Q r t Q 1 Q_~hort 1}'pe I An~.~~~.J~~arks) .... +._ ...__ .H _ ••••••••• _ •••••• ._ •• ~:=~".:'.I_

CHAPTER 8 : Electromagnetic Waves

1.1 Displacement CurrentThe current which comes into existence in theregion in which the electric field and the electricflux is changing with time. It is expressed by theformulae

Need for Displacement CurrentAmpere's circuital law for conduction currentduring charging of a capacitor was foundinconsistent. Therefore, Maxwell modifiedAmpere's circuital law by introducing the conceptof displacement current.

1.2 Maxwell's Equations ofElectromagnetic Waves

Maxwell's equations arc the basic laws ofelectricity and magnetism.These equations give complete description of allelectromagnetic interactions. On the basi of hisequations, Maxwell predicted the existence ofelectromagnetic waves.There are four Maxwell's equations which arcbelow:

(i) Gauss' Jaw in electrostatics.

f E·dS =!LEo

(ii) Gauss' law in magnetostatics,f B·dS=O

(iii) Faraday's law of electromagnetic inti uction,! E-dl = _ d

240 o Chopterwise CBSE Solved Popers PHYSICS

Important Characteristics of Electromagnetic Waves• The energy in electromagnetic wave is divided on average equally between electric and magnetic fields.

1 2 B2• Energy associated with an electromagnetic wave is U = -coE +--2 2110

• Electromagnetic waves also carry momentum.Linear momentum delivered to the surface,

Up=-.

cwhere,

U = total energy transmitted by electromagnetic wavesand c = speed of electromagnetic wave.

• Electromagnetic waves are not deflected by electric and magnetic fields.• Electromagnetic waves obey the principle of superposition. They show the properties of reflection,

refraction, interference, diffraction and polarization.

1.4 Electromagnetic SpectrumThe systematic sequential distribution of electromagnetic waves in ascending or descending order of frequencyor wavelength is known as electromagnetic spectrum. The range varies from 10-12 m to104 m, i.e. fromy-rays to radio waves.

Frequency(Hz) Wavelength(m)~~~ J1 ~~=~~/1021 Gammarays 10-12 /1020 10-11/1~9 1~~1018 )-'---- x-rays 1oil

10-810

17"0-7Ultraviolet "

1016i-:=:;::===r~~il:::===:;:t/10-81015.}- Visible ,10-51014 Infrared \30-41013 to-31012 Microwaves 10.-21011 10-',~1010 Shortradiowaves 1 \109 }- TelevisionandFMradio-{ 101 \108 102 \>--- AMradio----< ,107 103 \106 104 \105 Longradiowaves 10

5\

104 106

103 107

102101

The electromagnetic spectrum with common names far various port of it

400nm

Violet

450nmBlue

500nm

Green

550nm

Yellow

600nmOrange

650nm

Red

700 nm

CHAPTER B : Electromagnetic Waves 241

Different types of electrorm Llnetlc waves

Production DetectionType Wavelength Frequencyrange range (Hz)

Radio >0.1 m 104 to 109

wave

Microwave 0.1 m to 1 mm 109 to 1011

--Infrared 1 mm to700nm 3x1011 to4x1014

wave

Light 700 nm to 4 x 1014 to 8 x 1014400nm

Ultraviolet 400 nm to 1 nm 8 x 1014 to 8 x 1016

rays

Rapid acceleration anddeceleration of electrons inaerials. ..---.----.--~..Klystron valve or magnetronvalve.

Receiver's aerials

Point contact diodes

Vibration of atoms andmolecules.

Thermopile,Bolometer, infraredphotographic film

Electrons in atoms emit lightwhen they move from oneenergy level to a lower energylevel.

The eye, photocells,photographic film

Inner shell electrons in atomsmoving from one energy levelto a lower level.

Photocells,photographic film

X-rays 1 nm to 10 3 nm 1 x 1016to 3 X 1021 X-ray tubes or inner shellelectrons.

Photographic filmGeiger tubes,ionisation chamber

y-rays

PREVIOUS YEARS'EXAMINATION QUESTIONSTOPIC 1o 1 Mark Questions

1. Do electromagnetic waves carry energyand momentum? All Indio 2017

2. How is the speed of electromagneticwaves in vacuum determined by theelectric and magnetic fields? Delhi 2017

3. In which directions do the electric andmagnetic field vectors oscillate in anelectromagnetic wave propagating along theX-axis? All Indio 2017

4. Why are microwaves considered suitablefor radar systems used in aircraftnavigation? Deihl 2016

5. The charging current for a capacitor is0.25 A. What is the displacement currentacross its plates? Foreign 2016

6. To which part of the electromagneticspectrum does a wave of frequency5 x 1019Hz belong? All Indio 2014

7. To which part of the electromagneticspectrum does a wave of frequency3 x 1013Hz belong? All Indio 2014

8. Write the condition under which anelectron will move undeflected in thepresence of crossed electric and magneticfields. All Indio 2014C

9. Arrange the following electromagneticwaves in order of increasing frequency:y-rays, microwaves, infrared rays andultraviolet rays. Foreign 2014

1 O. Welders wear special goggles or facemasks with glass windows to protect theireyes from electromagnetic radiation.Name the radiations and write the rangeof their frequency. All Indio 2013

11. A capacitor has been charged by a DCsource. What are the magnitude ofconduction and displacement current,when it is fully charged? Delhi 2013

12. What are the directions of electric andmagnetic field vectors relative to eachother and relative to the direction ofpropagation of electromagnetic waves?All Indio 2012

13. Name the electromagnetic waves which(i) maintain the earth's warmth and

(ii) are used in aircraft navigation.Foreign 2012

14. A plane electromagnetic wave travels invacuum along Z- direction. What can yousay about the direction of electric andmagnetic field vectors? Delhi 2011

15. How are radio waves produced? All Indio 201116. Write two uses of microwaves. Foreign 201117. Write two uses of infrared rays. Foreign 201118. Write two uses of X-rays. Foreign 201119. What is the frequency of electromagnetic

waves produced by oscillating charge offrequency v = let Hz? Delhi 2011C

20. How are infrared waves produced? Whatis the range of their wavelength? Delhl2010C

21. Which of the following has the shortestwavelength?Microwaves, ultraviolet rays, X-rays.All Indio 2010

22. Name the part of electromagnetic spectrumwhose wavelength lies in the range of10-10 m. Give its one use. All Indio 2010

23. Arrange the following in descending orderof wavelength.X-rays, radio waves, blue light, infraredlight. All Indio 2010

24. Which part of electromagnetic spectrumhas largest penetrating power? Delhi 2010

25. Which part of electromagnetic spectrum isabsorbed from sunlight by ozone layer?Delhi 2010

26. Which part of electromagnetic spectrum isused in RADAR systems? Delhi 2010

27. Name the electromagnetic radiation usedto destroy cancer cells and write itsfrequency range. Foreign 2010

CHAPTER B : Electromagnetic Waves

28. Which part of the electromagneticspectrum is used in satellitecommunication? Foreign 2010

29. In what way, are the directions of theelectric and magnetic field vectorsrepresenting an electromagnetic waverelated to each other? Delhi 2010C

30. Express the velocity of propagation of anelectromagnetic waves in terms of thepeak value of the electric and magneticfields. All India 2010C

31. Name the electromagnetic waves used forstudying crystal structure of solids. Whatis its frequency range? All India 2009

32. Write the following radiations inascending order in respect of theirfrequencies X-rays, microwaves,ultraviolet rays and radio waves. Deihl 2009

33. Name the electromagnetic radiation towhich waves of wavelength in the range of10- 2 m belongs. Give one use of this partof electromagnetic spectrum. Delhi 2009, 2008

34. Name the electromagnetic radiationwhich can be produced by a klystron or amagnetron valve. Foreign 2009

35. Why is ozone layer on top of the stratospherecrucial for human survival? All India 2009C

o 2 Marks Questions36. How is the speed of electromagnetic

waves in vacuum determined by theelectric and magnetic fields? Delhi 2017

37. Identify the electromagnetic waves whosewavelengths vary as

(i) 10-12 m < f... < 10-8 m(ii) 10-3 m < f... < 10-1 m

Write one use for each. All India 2017

38. (i) How does oscillating charge produceelectromagnetic waves?

(ii) Sketch a schematic diagram depictingoscillating electric and magnetic fieldsof an EM wave propagating alongpositive Z-direction. Foreign 2014

243

39. (i) How are electromagnetic wavesproduced?

(ii) How do you convince yourself thatelectromagnetic waves carry energyand momentum? Oelhi 2013C

40. (i) Arrange the following electromagneticwaves in the descending order of theirwavelengths. Delhi 2013C

(a) Microwaves(b) Infrared rays(c) Ultraviolet radiation(d) y-rays

(ii) Write one use each of any two ofthem.

41. (i) An electromagnetic wave is travellingin a medium, with a velocity v = vi.Draw a sketch showing thepropagation of the electromagneticwave, indicating the direction of theoscillating electric and magneticfields.

(ii) How are the magnitudes of theelectric and magnetic fields related tovelocity of the electromagnetic wave?HOTS; Oelhl 2013; All India 2008C

42. A capacitor of capacitance C is beingcharged by connecting it across a DCsource along with an ammeter. Will theammeter show a momentary deflectionduring the process of charging? If so, howwould you explain this momentarydeflection and the resulting continuity ofcurrent in the circuit? Write theexpression for the current inside thecapacitor. All India 2012

43. When an ideal capacitor is charged by aDC battery, no current flows. However,when an AC source is used, the currentflows continuously. How does one explainthis, based on the concept of displacementcurrent? Delhi 2012

44. Explain briefly how electromagnetic wavesare produced by an oscillating charge? Howis the frequency of the electromagneticwaves produced related to that of theoscillating charge? Foreign 2012

244

45. Name the constituent radiation ofelectromagnetic spectrum which is usedfor

(i) aircraft navigation(ii) studying the crystal structure

Write the frequency range for each.Delhl2011C

46. Draw a sketch of a plane electromagneticwave propagating .along the Z-direction.Depict clearly the directions of electricand magnetic fields varying sinusoidallywith Z. All India 2011

47. Arrange the following electromagneticradiations in ascending order of theirfrequencies.(i) Microwaves (ii) Radio waves(iii) X-rays (iv) y-raysWrite two uses of anyone of these.Delhi 201D

48. How are X-rays produced? Write theirtwo important uses. Foreign 2010;Delhi 2D10

49. How are infrared rays produced? Writetheir two important uses.Foreign 2010; All Indio 201DC

50. How are microwaves produced? Writetheir two important uses. Foreign 2010

51,. Answer the following questions,(i) Optical and radio telescopes are

built on the ground while X-rayastronomy is possible only fromsatellites orbiting the earth. Why?

(ii) The small ozone layer on top of thestratosphere is crucial for humansurvival. Why? All India 2009

52. How does a charge, q oscillating atcertain frequency produceelectromagnetic waves? Sketch aschematic diagram depicting electric andmagnetic fields for an electromagneticwave propagating along the Z- direction.

Deihl 2009

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53. Name the electromagnetic radiations havingthe wavelength range from 1 mm to 700 nm.Give its two important applications.Foreign 2009

54. Name the electromagnetic radiation havingthe wavelength range from 10-1 m to10-3 m. Give its two importantapplications. Foreign 2009

55. Name the electromagnetic radiationshaving the wavelength range from 1 nm to10-3 nm. Give its two importantapplications. Foreign 2009

56. Identify the different types ofelectromagnetic radiations, which are used

(i) to kill germs,(ii) for physical therapy. All Indio 2009C

o 3 Marks Questions57. How are electromagnetic waves produced

by oscillating charges?Draw a sketch of linearly polarisedelectromagnetic waves propagating in theZ-direction. Indicate the directions of theoscillating electric and magnetic fields.Delhi 2016

58. Identify the part of the electromagneticspectrum which is

(i) suitable for radar system used inaircraft navigation,

(ii) produced by bombarding a metaltarget by high speed electrons.All Indio 2016

59. (i) Which segment of electromagneticwaves has highest frequency? How arethese waves produced? Give one use ofthese waves.

(ii) Which EM waves lie near the highfrequency end of visible part of EMspectrum? Give its one use. In whatway this component of light hasharmful effects on humans? Foreign 2016

CHAPTER 8 : Electromagnetic Waves

60. Name the parts of the electromagneticspectrum which is

(i) suitable for RADAR systems inaircraft navigations.

(ii) used to treat muscular strain.(iii) used as a diagnostic tool in medicine.Write in brief, how these waves can beproduced. All Indio 2015

61. Answer the following questions.(i) Name the waves which are produced

during radioactive decay of a nucleus.Write their frequency range.

(ii) Welders wear special glass goggleswhile working. Why? Explain.

(iii) Why are infrared waves often calledas heat waves? Give their oneapplication. Delhi 2014

62. Answer the following questions:(i) Name the EM waves which are

suitable for RADAR systems used inaircraft navigation. Write the rangeof frequency of these waves.

(ii) If the earth did not have atmosphere,would its average surfacetemperature be higher or lower thanwhat it is now? Explain.

(iii) An EM wave exerts pressure on thesurface on which it is incident.Justify. Delhi 2014

63. Answer the following questions:(i) Name the EM waves which are used

for the treatment of certain forms of'cancer. Write their frequency range.

(ii) Thin ozone layer on top ofstratosphere is crucial for humansurvival. Why?

(iii) Why is the amount of the momentumtransferred by the EM wavesincident on the surface so small?

Delhi 2014

64. State clearly how a microwave ovenworks to heat up a food item containingwater molecules.Why are microwaves found useful for theraw systems in aircraft navigation?Foreign 2011

245

65. Answer the following questions:(i) Show, by giving a simple example, how

EM waves carry energy and momentum.(ii) How are microwaves produced? Why

is it necessary in microwaves ovens toselect the frequency of microwaves tomatch the resonant frequency ofwater molecules?

(iii) Write two important uses of infraredwaves. Delhi 2014C

66. (i) Describe briefly how electromagneticwaves are produced by oscillatingcharges?

(ii) Give one use of each of the following.(a) Microwaves (b) Ultraviolet rays(c) Infrared rays(d) y-rays HOTS; All Indio 2011C

67. How are electromagnetic waves producedby oscillating charges? Why is it notpossible to produce electromagnetic wavesin the visible region with modern electroniccircuits in the laboratory? What is themethod of production of X-rays? Delhi 2009C

o Explanations1. Yes, electromagnetic waves carry energy and

momentum.

Momentum, p =!2 and energy density = ~£oE 2~ 2 rn

2. To determine speed of light in vacuum, we use theE Eformula, C = _0 = ~Bo Brms

where, Eo and Bo are maximum electric fieldand magnetic field component respectively ofelectromagnetic waves. (1)

3. E and B are perpendicular to direction ofpropagation of light. Also, direction ofpropagation is parallel to Ex B.Hence, E is along j or +Y-axis and B is along k or+Z-axis. (1)

4. On account of smaller wavelengths ofmicrowaves, they can be transmitted as beamsignals in a particular direction. Microwaves alsodo not bend around the corners of the obstaclescoming in their way. Thus, it is consideredsuitable for radar systems used in aircraftnavigation system. (1)

246

5. The displacement current is equal to 0.25 A. asthe charging current is 0.25 A. (1)

6. A wave of frequency 5 x 1019Hz belong to y-raysof electromagnetic spectrum. (1)

7. The frequency of 3 x 1013Hz belongs to theinfrared waves. (1)

B. v = ~, where v is the speed of electron.B rn

9. Microwaves < Infrared rays < Ultraviolet rays< y-rays. (1)

10. Welders wear special goggles or face mask withglass windows to protect their eyes fromultraviolet rays (UV rays). The range of UV raysis 4 x 1O-'m(400nm) to 6 x 10-10 m (0.6nm). (1)

11. Electric flux through plates of capacitor, Infrared> Blue light> X-ray is thedescending order of wavelength. (1)

24. y- rays have highest frequency range and hencehighest penetrating power. (1)

25. Ultraviolet ray of electromagnetic spectrum isabsorbed from sunlight by ozone layer. (1)

26. Microwaves. (1)27. 't: rays and frequency range is

1018 to 1022 Hz. [1)2B. Short radio waves A. > 01 m or v < 3 X 109 Hz are

used in satellite communication. (1)

29. The direction of propagation of electromagneticwaves is along the direction of Ex B, where Eand Bare electric and magnetic fields. (1)

30. Velocity of propagation of EM wave in terms ofpeak values of electric and field vectors.c= l.§J

Ia, I (1)31. X-rays are used to study the crystal structure of

solids. Frequency range of X - rays is1016 Hz to 1020 Hz. (1)

32. Radio waves < Microwaves < Ultraviolet rays< X-rays, is in ascending order of frequency. (1)

33. Microwaves have their wavelength in the range0.1 III to 1 mrn, so they are used in RADARcornmunlcat ion. (1)

CHAPTER 8 Electromagnetic Waves

34. Electromagnetic wave produced by klystron or amagnetron valve is microwave. (1)

35. Because ozone layer on top of stratosphere trapsmost of ultraviolet rays coming from the sun andprevents harmful impact of ultraviolet rays. (1)

36. When we charge a capacitor with the help of a cell,current flows all through the circuit except the partof circuit which lies between two plates ofcapacitor. There is no current found in that gap.But it was found that, an electric field exist insidethe plates therefore, indicating existence ofmagnetic field which leads to inconsistency in theAmpere's circuital law. To account this problem,concept of displacement current was introduced byMaxwell. He change the Ampere's circuital law(~B.dl = ~oI) t? a modified form which is givenbelow as,~B.dl = ~oI + ~oId where, Id is displacementcurrent.This is known as Ampere's-Maxwell law,According to this law, displacement current is arisedue to the changing electric field which in turncreates magnetic field between the plates.It can be shown that, Id = Eo dIPE

dt (1)

37. (i) 1O-12m-1O-8m=.OIA-100A.-X-ray.It is used in crystallography.

(il) IO-3m_10-1m= 0.1 cm-lO ern -Radio waves.It is used in radio communication.

38. (i) The oscillating charge produces an oscillatingelectric field and an oscillating electric fieldproduces magnetic field which is then producesan oscillating emf. An oscillating voltage (emf)produces an oscillating magnetic field and soon. In this way. the oscillating charges producean electromagnetic waves. (1)

(ii) The propagation of electromagnetic wave isshown in figure.

y

E B E B

BZ Direction of

propagation

A plane electromagnetic wove travelling along X -cxls (1)

(1)

247

39. (i) A changing electric field produces a magneticfield and a changing magnetic field producesan electric field (Faraday's law), the result isa wave of electric and magnetic fields thatcan propagate through space. Thesepropagating fields are called aselectromagnetic waves.Hence, an electromagnetic wave originatesfrom the oscillating charge. The energycarried by the wave comes from the sourcewhich makes the charge oscillating. (1)

(ii) According to the quantum theory,electromagnetic radiation is made up ofmassless particles called photons.Momentum of the photon is expressed as

Ep=-c .(1)

where. p = momentum, E = energyand c = speed of light.

Thus, I am convinced that electromagnetic wavecarry energy and momentum.(i) The decreasing order of wavelengths of

electromagnetic waves areMicrowaves > Infrared > Ultraviolet

> y-rays (1)(ii) Microwaves They are used in RADAR

devices.y-rays It is used in radio therapy. (1)

40.

41. We are taking direction of electric field (E) andmagnetic field (8) in such a way that these arefollow the right handed system of axes.

(i) Given that velocity, v = v i and electric field Ealong Y-axisand magnetic field B alongZ-axis. (1)

(ii) Speed of electromagnetic wave can be givenas c = Eo =!!..

Bo B

where, Eo and Bo = peak value of E and B, E andB are instantaneous value of E and B. (1)

42. The ammeter will show the momentarydeflection. (1/2)This momentary deflection occurs due to thefact that the conduction current flows throughconnection wires during the charging ofcapacitor. This lead to gathering of charge attwo plates and hence varying electric field ofincreasing nature is produced between theplates which in turn produce displacement

248

current in space between two plates. Thismaintains the continuity with the conductioncprJO!nt. "

\fJ Ie = ID (1)

The current inside the capacitordc!>E

Displacement current, ID and I D = Eo --dt (1/2)

43. The capacitive reactance,I I

Xe=-=--·wC 27tfC

In AC, Xefor ideal capacitor is unique and finiteand hence current flows. (1)In DC circuit momentarily current flows duringcharging of capacitor.This conduction current will be equal to thedisplacement current flowing between the platesof capacitor due to varying electric field, i.e.Conduction current = Displacement current

dchand ID = Eo--dt (1)

44. An oscillating charge is considered as theaccelerating charge. This produces an oscillatingelectric field in space, which produces anoscillating magnetic field, that in turn againproduces oscillating electric field and so on. Theseoscillating electric and magnetic fields thus keepson regenerating each other as the wave propagatesthrough the space. The frequency of theelectromagnetic waves naturally equals thefrequency of oscillation of the charge. (2)

45. (i) Microwaves are used for aircraft navigation,their frequency range is 109 Hz to 1012 Hz (1)

(ii) X-rays are used to study crystal structure, theirfrequency range is 1016 Hz to 1020 Hz. (1)

46. The direction of propagation of electromagnetic"wave is perpendicular to both electric field vector Eand magnetic field vector B, i.e. in the direction ofExB.This can be seen by the diagram given below

X

E

y

Here, electromagnetic wave is along theZ-direction which is given by the cross product ofEand B. (2)

o ehapterwise eBSE Solved Papers PHYSICS

47. Ascending order of the frequenciesRadio waves < Microwaves < X-rays < y-rays. (2)Uses of X-rays.(i) To cure untracable skin diseases and maligant

growths (Radio therapy).(ii) For testing the uniformity of insulating material.

48. X-rays can be produced by colliding fast movingelectron beam on metal target. (1)Uses(i) Medical diagnosis.

(ii) Study of crystal structure. (1)49. Source Infrared waves are produced by hot

objects and vibration of atoms and molecules. (1)Uses(i) In remote control of TV, VCR ete.

(ii) In photography in thin mist. (1)

50. Microwaves are produced by oscillating currentin vacuum tubes like klystrons, magnetrons. (1)For applications Refer to Ans, 16. (1)

51. (i) Because X-rays are shorter than ultravioletrays and can pass through many forms ofmatter whereas light wave or radio wavecannot pass. Therefore, X-ray astronomy ispossible from satellite. (1)

(ii) Refer to Ans. 35. (1)

52. Due to an oscillating charged particle varyingelectric current is associated with the particlehence the magnetic as well as electric field willbe associated which is varying in nature thisproperty is utilised in electromagnetic wave,

z

The oscillation charge accelerates and henceequivalent to varying electric current which in turnchanges the magnetic field. This lead to productionof electric field. Thus, oscillatory charge producestime varying electric and magnetic fields. Hence,electromagnetic waves produces. (1)For diagram Refer to Ans. 46. (1)

53. Infrared waves have the wavelength betweenI mm to 700 nm. (1)For application Refer to Ans. 17. (1)

54. Microwaves have the wavelength between 10-1 mto 10-3 m (1)For application Refer to Ans. 16. (1)

55. X-rays have the wavelength between I nm and10-3 nm (1)For application Refer to Ans. 18. (1)

56. (i) y-rays (ii) Infrared waves (2)

CHAPTER B Electromagnetic Waves

57. An accelerating charge produces electric andmagnetic fields, both changing with time andspace sinusoidally. An oscillating L-C circuit canproduce electromagnetic waves of frequency ascharge oscillates across the capacitor's plates inthis circuit. The frequency of oscillation is given by

1v=---21t.JLC (1'10)x

--_Wave propagation

y

58.

A linearly palarised EM wavesIn this diagram, we see that permanent curveshows electric field E and dotted curve showsmagnetic field B. They change continuously.Electromagnetic waves so produced have thedirection perpendicular to E and B. (1'10)(i) Microwaves are suitable for radar system used

in aircraft navigation. (1)(ii) X-rays are produced by bombarding a metal

target by high speed electrons. (2)

(i) Gamma rays has the highest frequency in theelectromagnetic waves. These rays are of thenuclear origin and are produced in thedisintegration of radioactive atomic nuclei and inthe decay of certain subatomic particles. They areused in the treatment of cancer and tumours. (1)

(ii) Ultraviolet rays lie near the high-frequency endof visible part of EM spectrum. These rays areused to preserve food stuff. The harmful effectfrom exposure to ultraviolet (UV) radiation canbe life threatening, and include prematureaging of the skin, suppression of the immunesystems, damage to the eyes and skin cancer. (2)

(i) The EM waves suitable for RADAR system ismicrowaves the range of frequency is 3xl011to 1 Xl08 Hz.These rays are produced by special vacuumtubes, namely klystrons, magnetrons andGunn diodes.

(ii) Infrared waves are used to treat muscularstrain. These rays are produced by hot bodiesand vibration of molecules and atoms. (1)

(iii) X-rays are used a diagnostic tool in medicine.These rays are produced when high energyelectrons are stopped suddenly on a metal ofhigh atomic number. (1)

(i) y-rays are produced during radioactive decayof a nucleus. Its frequency range is from3 x 1018 to 5 X 1022 Hz.

59.

60.

61.

249

(ii) Refer to Ans. 10. (1)(iii) Infrared rays are called heat waves because

they vary temperature and are produced byvibration of atoms and molecules. (1/2)They are used in physical therapy and'weatherforecasting. (112)

62. (i) Refer to Ans. 60 (i). (1)(ii) The temperature of the earth would be lower

because the greenhouse effect of theatmosphere would be absent. (1)

(iii) An EM waves has momentum,i.e. _ Energy (E)

p - Velocity of light (c)

When it is incident upon a surface it exertspressure on it.

63. (i) y-rays are used for the treatment of certainforms of cancer. Its frequency range is3 x 1019 Hz to 5 X 1022 Hz.

(1)

(1)

(1)

(ii) The thin ozone layer on top of stratosphereabsorbs most of the harmful ultraviolet rayscoming from the sun towards the earth. Theyinclude UVA, UVB and uve radiations, whichcan destroy the life system on the earth.Hence, this layer is crucial for human survival.

(1)

(iii) An electromagnetic wave transports linearmomentum as it travels through space. If anelectromagnetic wave transfers a total energy Uto a surface in time t, then total linearmomentum delivered to the surface is

Up=-

c[For complete absorption of energy Uj

If the wave is totally reflected, the momentumdelivered will be 2U / c. because the momentum ofthe wave will change from p to -po (1)

'"64. Microwaves are short wavelength radiowaves whichare produced by vacuum tubes by rapid acceleration.This kinetic energy of molecules raises thetemperature of any food containing water.Microwaves are short wavelength radio waves, withfrequency of order of GHz. Due to short wavelength,they have high penetrating power with respect toatmosphere and less diffraction in the atmosphericlayers. So, these waves are suitable for the radarsystems used in aircraft navigation. (1)In microwave oven, the frequency of the.microwaves is selected to match the resonantfrequency of water molecules so that energy from.ie waves get transferred efficiently to the kinetic

energy. (2)(1)

250

65. (i) Consider a plane perpendicular to the directionof propagation of the wave. An electric charge,

. on the plane will be set in motion by theelectric and magnetic fields of EM wave,incident on this plane. This illustrates that EMwaves carry energy and momentum. (1)

(ii) Microwaves are produced by special vacuumtube like the klystron, magnetron and Gunndiode. The frequency of microwaves is selectedto match the resonant frequency of watermolecules, so that energy is transformedefficiently to the kinetic energy of themolecules. (1)

(iii) (a) Associated with the greenhouse effect.(b) In remote switches off household electrical

appliances. (1)

66. (i) For electromagnetic waves Refer to Ans. 52.(1)

o ehopterwise eBSE Solved Papers PHYSICS

(ii) For uses of microwaves, ultraviolet raysand infrared rays Refer to Ans.16 and 17. (1)Application of ultraviolet rays For checkingthe mineral samples through the property ofultraviolet rays causing fluorescence.Use of y-rays

In the treatment of cancer and tumours, (1)67. The oscillating charge accelerates which in turn

produces time varying current. This producesvarying magnetic field. The varying magneticfield induces the varying electrical field. Thus,varying electric field and varying magnetic fieldbecomes source of each other and hence,electromagnetic waves comes into existence. (1)Electronic circuit can not produces electromagneticwaves in the visible region because their frequencyis much higher than that of microwaves.For X-ray Refer to Ans. 48. (2)

Value Based Questions (From Complete Chapter)o 4 Marks Questions

1. Mr. Kamal and Jayant were playingfootball in the ground. In the middle ofthe match, Kamal fell down as there wasa cramp in his right leg. Jayant rushedtowards him and picked him up and toldhim not to move. They went to the nearbyhospital. Doctor examined his legs andadvised for an X-ray test for theconfirmation of fracture.

(i) What are the value Jayant showshere?

(ii) How are X-ray produced?(iii) Can you give one another application

of X-ray?Ans. (i) Jayant is helpful, caring and has

presence of mind to deal with adversesituation.

(ii) X-ray is produced by thebombardment of a metal target byhigh energy electrons in X-ray tubes.

(iii) It is used in the determination ofstructure of solids by X-raysdiffraction method or in engineeringfor detecting faults, cracks, flows andholes in the finished metal products.

2. Shyam and his younger brother were atthe restaurant, It was very clean there.None of the flies and insects were there.His younger brother asked him about thefluorescent UV lamp present at thecorner. Shyam explained the functioningof UV lamp, how flies and insects gettrapped by it?

(i) What are the values shown byShyam?Give the source of UV-rays?Give the harmful effect of UV-rays.Shyam is intelligent and has clarityin explaining facts. He is also awareabout health.

(ii) UV-rays are produced by the sun andis trapped by ozone layer. Actuallyinner shell electrons in atoms movingfrom higher energy level to a lowerenergy level.

(iii) UV-rays can cause skin cancer, so itis carcinogenic.

(ii)(iii)

Ans. (i)