-
- 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
o Chopterwise CBSE Solved Popers PHYSICS
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)