CBSE Class 12 Physics HOTs Electromagnetic Waves

 UNIT 05

(EM waves)

1 MARK QUESTIONS:

ONE MARK QUESTIONS

Question. The frequency of oscillation of the electric vector of a certain electromagnetic wave is 5x1014 Hz. What is the frequency of oscillation of the corresponding magnetic field vector and to which part of the electromagnetic spectrum does it belong?
Answer : 5x1014 Hz, visible region.

Question. Name the electromagnetic radiations used for studying the crystal structure of solids and write its frequency range
Answer : X- rays, frequency range 1017 to 1020 HZ

Question. The speed of an electromagnetic wave in a material medium is given by v = 1/√µε, µ being the permeability of the medium and € its permittivity. How does its frequency change?
Answer : No change

Question. How are X-rays produced?
Answer : X-rays are produced when high energetic electron beam is made incident on a metallic target of high melting point and high atomic weight.

Question.Which of the following has the shortest wavelength?
Microwaves, Ultraviolet rays, X-rays
Answer : X-rays

TWO MARKS QUESTIONS

Question. Identify the following electromagnetic radiations as per the frequencies given below:
(a) 1020 HZ (b) 109 HZ (c) 1011 HZ
Write one application of each.
Answer : (a) gamma radiation, for treatment of cancer
(b) Radio waves, for broadcasting radio programmes to long distances.
(c) Micro waves, for cooking in microwave oven.

Question. Write the order of frequency range and one use of each of the following electromagnetic radiations
(a)Microwaves (b) Ultraviolet rays (c) Gamma rays
Answer : (a) Microwaves: 3X 1011- 1X 108Hz. These are suitable for the radar systems, used in aircraft navigation.
Ultraviolet rays: 1X106 - 8X1014Hz. They are used to detect invisible writing, forged documents and finger prints.
Gamma rays: 5X 1023- 3X1019Hz. For the treatment of cancer cells.

Question. Arrange the following electromagnetic eaves in the order of their increasing wavelength:
Gamma rays, microwaves, x rays, radio waves.
How are infra-red waves produced? What role does infra-red radiation play in (i) maintaining the Earth’s warmth and (ii) physical therapy?
Answer : Gamma rays, x rays, microwaves, radio waves.
Infra-red waves are produced by the vibration of atoms and molecules
(i) The earth radiates infrared waves which are reflected by the gases in the lower atmosphere. This phenomenon, called greenhouse effect, keeps the earth warm.
(ii) Infrared lamps in the treatment of muscular complaints.

Question. Why are infra-red radiations referred to as heat waves? Name the radiations which are next to these radiations in the electromagnetic spectrum having (i) shorter wavelength (ii) longer wavelength.
Answer : Infrared waves are produced by hot bodies and molecules, so are referred to as heat waves.
(i)Em wave having short wavelength than infrared waves are visible, UV, X-rays and ϒ-rays.
(ii) Em wave having longer wavelength than infrared waves are microwaves, short radio waves, television and FM radio.

Question. Does the colour of radiation depend on its frequency or on wavelength. 
Answer : Frequency.

Question. What physical quantity is the same for X-rays of wavelength 1A, green light of wavelength 5500A0 & radiation of wavelength 21cm? 
Answer : Speed.

Question. Electromagnetic radiations with wavelength:
1): λ1 are used to kill germs in water purifiers.
2): λ2 are used in T.V communication system. 
Answer : 
1) λ₁ corresponds to ultraviolet spectrum.
2) λ₂ corresponds to radio waves. 

2 MARK QUESTIONS:

Question. Why stationary charges & constant currents do not produce electromagnetic waves? 
Answer : A stationary charge & constant current produce a constant electric field & constant magnetic field respectively. A constant electric field can't generate a magnetic field likewise a constant magnetic field cannot generate a electric field. Hence, EM waves can't be produced.

Question. If the electric field that constitutes an electromagnetic wave conservative? Justify your answer. 
Answer : No, the electric field produced by a time varying magnetic field is non conservative. So that electric field that constitutes the EM waves is nonconservative.

Question. The radio waves, the infrared, the visible ray are EM radiations. Then how are they different from each other?
Answer : They are different because the way they interact with matter is different. Interaction depends on the energy of the EM waves, which in turn depends upon its frequency (E=hv).  

1.Why is the quantity Ε₀ dφ_E/dt called the displacement current?

8. Scientists put x-ray astronomical telescope on the artificial satellite orbiting above the earth's atmosphere whereas they build optical and radio-telescopes on the surface of the earth. Why ?

X-rays have very high frequency and much smaller wavelength. These rays get absorbed by the earth's atmosphere. On the other hand, optical (visible) radiations and radio-waves can pass through the atmosphere. That is why optical and radio telescopes can be installed on the earth's surface.

 

9. For an electrom agnetic wave, write the relationship between amplitude of electric and m agnetic fields in free space.

[H in ts If Eo is the amplitude of an e lectric fie ld and B_o is the amplitude o f the associated m agne tic fie ld in free space then c=E₀/B₀ where c is the speed o f light

in free space i.e. 3x10⁸ m /s.

 

10. The charging current for a capacitor is 0.25 A. What is the displacement current across its plates?

Hints: Displacement current = Charging current = 0.25 A]

 

11. What is a ground wave? How does it differ from a sky wave?

Ans. A signal emitted by an antenna from a certain point can be received at another point of the surface of the earth in two ways. The wave which travels directly following the surface of the

earth is called ground nave. The wave that can reach the same point after being reflected from the ionosphere is called sky wave.

 

12. Why short wave communication over long distances is not possible via ground waves?

|Hints: Because the wave gets attenuated.]

 

13. A re conduction and disp lacement currents the same?

[H in t: N o; they are different but they a re equal .

 

14. It is necessary to use satellites for long d istance TV transmission. W hy? It is so because te levision signals a re not properly reflected by the ionosphere. Therefore, for reflection of signa ls satellites a re needed as reflection is effected by sate llites.

 

15. O ptical and radio telescopes a re built on g round but X-ray astronomy is possible only from sate llites o rbiting the earth . W hy?

A tmosphere absorbs X-rays, while visible and radiowaves can penetrate it. That is why optica l and rad io te lescopes can work on earth 's surface but X -ray astronomical telescopes m ust be used on satellites o rbiting the earth.

 

16. If the earth did not have an atmosphere, would its average surface temperature be h igher or lower than what it is now?

Ans. The temperature of the earth would be lower because the g reen house e ffect of the atmosphere would be absent.

.

17. W hat is the equation for the speed o f e lectromagnetic waves in free space?

18. The wavelength o f e lectro magnetic radiation is doubled. W hat w ill happen to the energy o f the photon?

T he energy will be halved because frequency will be halved.

 

19. Name the part o f e lectromagnetic spectrum to which waves o f wavelength (i) 1A° and (ii) 10^-2 m b elong. U sing the relation lT = (0.29 cm) K, obta in the characteristic ke lvin temperature corresponding to these two wavelengths.

(i) X -rays, (ii) Microwaves

Again, (i) T= 0.29cm/1x10^-10cm=29x10^-8k            (ii)    T=0.29cm/1cm=0.29k

 

20. G ive a simple a rgument to suggest that an accelerated charge m ust emit electromagnetic radiation

Ans. W hen charge m oves with constant velocity, the m agnetic field does not change with time. So, it cannot p roduce an electric field. W hen charge is accelerated, both electric and magnetic fie lds change w ith time and space, one becoming a source of another. This g ives rises to electromagnetic wave.

 

21. Show that the average energy density o f the E field equals the average energy density of the B field.

Ans. Energy density in E field,

22. On what factors does its velocity in vacuum depend?

Ans. Electromagnetic waves consist o f sinusoidal variation of e lectric and magnetic field vectors. The fie ld vectors vibrate with the same frequency and are in the same phase. The field vectors and the d irection of propagation a re all mutually perpendicular. The velocity o f e lectromagnetic waves in vacuum depends upon absolute permeability μ_o and abso lute permittivity e₀.N ote that c

(i) G auss's law (ii) No particular name

(iii) Faraday's law (iv) Ampere 's law.

 

24. E lectromagnetic waves with wavelength

(i)λ₁are used to treat m uscu lar strain

(ii) λ₂ a re used by a FM rad io station for broadcasting

(iii)λ₃ a re used to detect fractu re in bones

(iv) λ₄ a re absorbed by the ozone layer of the a tmosphere.

 

Identify and name the part o f the e lectromagnetic spectrum to which these radiations b elong. A rrange these wavelengths in decreasing o rder o f m agnitude.

 

Ans. (i) λ1 -»infrared (ii) λ2 -» radiowaves

(iii) l3 — > X-rays (in) λ4 —> u ltra-violet rays λ2 >λ1 > λ4 > λ3.

 

25. If you find closed loops o f B in a region in space, does it necessarily m ean that actual charges a re flowing across the area bounded by the loops?

Ans. Not necessarily. A d isplacement current such as that b etween the plates o f a capacitor that is being charged can a lso p roduce loops of B •

 

26. A closed loop o f B is p roduced by a changing e lectric field. Does it necessarily mean that E and dt/dE are non-zero a t a ll p oints on the loop and in the area enclosed by the loop?

Ans. Not necessarily. A ll that is needed is that the total e lectric flux through the area enclosed by the loop should vary in time. The flux change m ay a rise from any portion of the a rea. E lsewhere E o r dt/dE m ay be zero. In particular, there need be no electric field at the points which m ake the loop.

 

27. W hy is it that induced e lectric fields due to changing magnetic flux a re more readily observable than the induced magnetic fields due to changing electric fields?

Ans. The m agnitude of the magnetic field due to d isplacement current is too small to be easily observable. This e ffect can o f course be increased by increasing the displacement current. [In an AC circuit, this can be done by increasingw.]

On the o ther hand, the effect of induced e lectric field due to changing m agnetic flux can be increased simply by taking m ore and m ore number of turns in the co il. The induced em fs in d ifferent turns o f the same coil add up in series.

 

28. A variable-frequency AC source is connected to a capacitor. W ill the displacement current increase o r decrease with increase in frequency?

Ans. Increase in frequency causes decrease in impedance o f the capacitor and consequent increase in the current which equals d isplacement current between the p lates.

 

29. Some scientists have predicted that a g lobal nuclear war on the earth would be followed by a severe 'nuclear winter' with a devastating e ffect o n life on earth. W hat might be the basis o f this p rediction?

Ans. The clouds p roduced by a g lobal nuclear war would perhaps cover substantial parts o f the sky p reventing solar light from reaching m any parts o f the globe. T his would cause a 'winter'.

30. What is the contribution of the Greenhouse effect towards the surface temperature of the earth?

Ans. The infrared radiation emitted by the earth's surface keeps the earth warm. In the absence of this effect, the surface temperature of earth would be lower.

 

31. Why the small ozone layer on top of the stratosphere is crucial for human survival?

Ans. The small ozone layer on the top of the stratosphere absorbs ultraviolet radiations, yrays etc. from the sun. It also absorbs cosmic radiations. So, these radiations, which can cause genetic damage to the living cells, are prevented from reaching the earth. Thus, the small ozone layer on top of the stratosphere is crucial for human survival.

 

32. A plane electromagnetic wave travels in vacuum along z-direction. What can you say about the directions of its electric and magnetic field vectors? If the frequency of the wave

is 30 MHz, what is the wavelength?

Ans. E and B lie in the x-y plane and are mutually perpendicular. Wavelength,

λ=3x10⁸/30x10⁶

 

33. Given below are some famous numbers associated with electromagnetic radiation in different contexts in physics. State the part of the e m spectrum to which each belongs.

(i) 21 cm (wavelength emitted by atomic hydrogen in interstellar space).

(ii) 1057 MHz [frequency of radiation arising from two close energy levels in hydrogen; known as Lamb shift].

(Hi) 2.7 K temperature associated with the isotropic radiation filling all space-thought to be

a relic of the 'big-bang’ origin of the universe.

(iv) 5890 A - 5896 A [double lines of sodium].

(v) 14.4 keV [energy of a particular transition in 57Fe nucleus associated with a famous high resolution spectroscopic method (Mossbauer spectroscopy).

Ans. (i) Radio (short wavelength end) (ii) Radio (short wavelength end) (Hi) Microwave (iv)

Visible (Yellow) (v) X-rays (or soft y-ray) region.

 

34. Electromagnetic waves in a cavity with conducting walls can exist only in certain modes i.e., they cannot exist, for example, with any arbitrary wavelength. Suggest a simple reason why this should be so.

Ans. The waves must satisfy a boundary condition. The electric field should be zero on the walls of the conductor. This restricts the possible modes. [It is something like the restricted

modes of a string fixed at two ends.]

 

 Write the order of frequency range and one use of the following e. m radiations.

i)Microwaves ii) Ultra-violet rays iii) gamma rays

a. i) Microwaves :- 109 to 1012 Hz approx used in radars/microwave ovens

ii) Ultra violet rays:-1015 to 10 18 Hz approx. sterilizing the medical instruments

iii) gamma rays:- 10 18 to 10 22 Hz in radio therapy ,treating cancer and tumours.

 

8. How are the following e.m radiations produced, Mention one used of them.

i) radio waves ii ) infra red rays iii) x rays

a. i) radio waves :produced by oscillating circuits having inductor and capacitor. Used in radio and TV communication.

ii)infra red rays: produced by all hot bodies treating muscular strains/ solar appliances etc.,

iii) gamma rays: produced when high energy electrons bombards metals. . to detect fracture in bones/ study crystal structure

14.The small ozone layer on the top of the atmosphere is crucial for human survival. Why?

The ozone in the atmosphere is confined to the ozone layer, some 50-80 km above the ground. The ozone layer blocks the passage of the ultra-violet radiations; x-rays and y-rays from the solar and other extraterrestrial sources and effectively protects us from the dangerous and harmful portions of solar radiations as they cause genetic damages to living cells. Practically all radiations of wave length less than 3 x 10^-7m are absorbed by the ozone layer. This explains why ozone layer on the top of the atmosphere is crucial for human survival.

 

15.How does "Green House Effect" affect the temperature of the earth's surface ?

Green house effect serves to keep the earth's surface warm at night.

 

16.How do we make television broadcasts for larger coverage and for long distance?

By using (i) tall antennas which is familiar landmark in many cities and (ii) using artificial satellites —called geostationary satellites. Since television signals are of high frequency and are not reflected by ionosphere so we use satellites to get them reflected & transmission of TV signals can be used for larger coverage as well as for long distance.

 

17. Scientists put x-ray astronomical telescope on the artificial satellite orbiting above the earth's atmosphere whereas they build optical and radio-telescopes on the surface of the earth. Why ?

X-rays have very high frequency and much smaller wavelength. These rays get absorbed by the earth's atmosphere. On the other hand, optical (visible) radiations and radio-waves can pass through the atmosphere. That is why optical and radio telescopes can be installed on the earth's surface.

 

18. For an electromagnetic wave, write the relationship between amplitude of electric and magnetic fields in free space.

[Hints If E_o is the amplitude of an electric field and B_o is the amplitude of the associated magnetic field in free space then c= E₀/B₀

where c is the speed of light in free space i.e . 3x10⁸ m /s.

 

19. The charging current for a capacitor is 0.25 A. What is the displacement current across its plates?

Hints: Displacement current = Charging current = 0.25 A]

 

20. Why short wave communication over long distances is not possible via ground waves?

|Hints: Because the wave gets attenuated.]

 

21. Are conduction and displacement currents the same?

[Hint: No; they are different but they are equal .

 

22. It is necessary to use satellites for long distance TV transmission. Why?

It is so because television signals are not properly reflected by the ionosphere. Therefore, for reflection of signals satellites are needed as reflection is effected by satellites.

 

23. Optical and radio telescopes are built on ground but X-ray astronomy is possible only from satellites orbiting the earth. Why?

Atmosphere absorbs X-rays, while visible and radiowaves can penetrate it. That is why optical and radio telescopes can work on earth's surface but X-ray astronomical telescopes must be used on satellites orbiting the earth.

 

24. If the earth did not have an atmosphere, would its average surface temperature be higher or lower than what it is now ?

Ans. The temperature of the earth w ould be lower because the green house effect of the atmosphere w ould be absent.

 

25. What is the equation for the speed of electromagnetic waves in  free space?

26. The w avelength of electro magnetic radiation is doubled. What w ill happen to the energy of the photon?

The energy w ill be halved because frequency w ill be halved.

 

27. Name the part of electromagnetic spectrum to which waves of wavelength (i) 1A° and (ii) 10-2 m belong. Using the relation λT = (0.29 cm) K, obtain the characteristic kelvin temperature corresponding to

these tw o w avelengths.

(i) X -rays, (ii) M icrowaves

 

29. On w hat factors does its velocity in vacuum depend?

Ans. Electromagnetic w aves consist of sinusoidal variation of electric and magnetic field vectors. The field vectors vibrate with the same frequency and are in the same phase. The field vectors and the direction of propagation are all mutually perpendicular. The velocity