CBSE Class 12 Physics Electromagnetic Waves MCQs Set F

Question: The part of the spectrum of the electromagnetic radiation used to cook food is 
(a) ultraviolet rays
(b) cosmic rays
(c) X-rays
(d) microwaves 
Answer: d

Question: A plane electromagnetic wave travels in vacuum along z-direction. If the frequency of the wave is 40 MHz then its wavelength is
(a) 5 m
(b) 7.5 m
(c) 8.5 m
(d) 10 m
Answer: b

Question: A radio can tune to any station in 7.5 MHz to 12 MHz band. The corresponding wavelengthband is
(a) 40 m to 25 m
(b) 30 m to 25 m
(c) 25 m to 10 m
(d) 10 m to 5 m 
Answer: a

Question: About 6% of the power of a 100 W light bulb is converted to visible radiation. The average intensity of visible radiation at a distance of 8 m is (Assume that the radiation is emitted isotropically and neglect reflection).
(a) 3.5 × 10^–3 W m^–2
(b) 5.1 × 10^–3 W m^–2
(c) 7.4 × 10^–3 W m^–2
(d) 2.3 × 10^–3 W m^–2
Answer: c

Question: Which of the following rays is not an electromagnetic wave?
(a) X-rays
(b) g-rays
(c) b-rays
(d) Heat rays 
Answer: d

Question: A plane electromagnetic wave of frequency 25 MHz travels in free space along x-direction.
At a particular point in space and time, electric field E = 6.3 V m^–1. The magnitude of magnetic field B at this point is
(a) 1.2 × 10^–6 T
(b) 1.2 × 10^–8 T
(c) 2.1 × 10^–6 T
(d) 2.1 × ^10–8 T
Answer: d

Question: The ratio of contributions made by the electric field and magnetic field components to the intensity of an electromagnetic wave is 
(a) c : 1
(b) c² : 1
(c) 1 : 1
(d) √c :1 
Answer: c

Question: The source of electromagnetic waves can be charge, when
(a) moving with a constant velocity
(b) moving in a circular orbit
(c) falling in an electric field
(d) both (b) and (c) 
Answer: d

Question: Which waves are used in sonography ?
(a) Microwaves
(b) Infrared rays
(c) Radio waves
(d) Ultrasonic waves
Answer: d

Question: Light with an energy flux of 18 W cm^–2 falls on a non-reflecting surface at normal incidence.
If the surface has an area of 20 cm², the average force exerted on the surface during a 30 minute time span is
(a) 2.1 × 10^–6 N
(b) 1.2 × 10^–6 N
(c) 1.2 × 10^–6 N
(d) 2.1 × 10^–6 N
Answer: b

Question: Radiations of intensity 0.5 W m^–2 are striking a metal plate. The pressure on the plate is
(a) 0.166 × 10^–8 N m^–2
(b) 0.332 × 10^–8 N m^–2
(c) 0.111 × 10^–8 N m^–2
(d) 0.083 × 10^–8 N m^–2 
Answer: c

 Question: An electromagnetic wave of frequency u = 3 MHz passes from vacuum into a dielectric medium with permittivity e = 4. Then
(a) wavelength and frequency both become half.
(b) wavelength is doubled and frequency remains unchanged.
(c) wavelength and frequency both remain unchanged.
(d) wavelength is halved and frequency remains unchanged. 
Answer: d

Question: One requires 11 eV of energy to dissociate a carbon monoxide molecule into carbon and oxygen atoms. The minimum frequency of the appropriate electromagnetic radiation to achieve the dissociation lies in
(a) visible region.
(b) infrared region.
(c) ultraviolet region.
(d) microwave region. 
Answer: c

Question: The electric field associated with an electromagnetic wave in vacuum is given by →E = 40 cos(kz − 6 × 10⁸t)i^{^} , where E , z and t are in volt per meter, meter and second respectively.
The value of wave vector k is
(a) 2 m^–1
(b) 0.5 m^–1
(c) 6 m^–1
(d) 3 m^–1  
Answer: a

Question: An electromagnetic wave radiates outwards from a dipole antenna, with the amplitude of its electric field vector E₀. The electric field which transports significant energy from the source falls off as
(a)1/r³
(b 1/r²
(c) 1/r
(d) r/
Answer: c

Question: A plane electromagnetic wave is incident on a material surface. The wave delivers momentum p and energy E. Then
(a) p ≠ 0, E ≠ 0
(b) p = 0, E = 0
(c) p = 0, E ≠ 0
(d) p ≠ 0, E = 0 
Answer: a

Question: The amplitude of the magnetic field of a harmonic electromagnetic wave in vacuum is B0 = 510 nT. The amplitude of the electric field part of the wave is
(a) 120 N C^–1
(b) 134 N C^–1
(c) 510 N C^–1
(d) 153 N C^–1  
Answer: d

 Question: The decreasing order of wavelength of infrared, microwave, ultraviolet and gamma rays is
(a) microwave, infrared, ultraviolet, gamma rays
(b) infrared, microwave, ultraviolet, gamma rays
(c) gamma rays, ultraviolet, infrared, microwaves
(d) microwaves, gamma rays, infrared, ultraviolet
Answer: a

Assertion & Reasoning Based MCQs

two statements are given-one labelled Assertion (A) and the other labelled Reason (R).
Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below.
(a) Both A and R are true and R is the correct explanation of A
(b) Both A and R are true but R is NOT the correct explanation of A
(c) A is true but R is false
(d) A is false and R is also false

Question: Assertion : Only microwaves are used in radar.
Reason : Because microwaves have very small wavelength. 
Answer: a

Question: Assertion (A) : Radio waves cannot be diffracted by the buildings.
Reason (R) : The wavelength of radio waves is very small.
Answer: d

Question: Assertion (A) : Light can travel in vacuum whereas sound cannot do so.
Reason (R) : Light has an electromagnetic wave nature whereas sound is mechanical wave.
Answer: a

Question: Assertion (A) : The electric field and magnetic field have equal average values in linearly polarised plane em wave.
Reason (R) : The electric energy and magnetic energy have equal average values in linearly polarised plane em wave.
Answer: b

Question: Assertion (A) : Velocity of light is constant in all media.
Reason (R) : Light is an electromagnetic wave which has constant velocity in all media.
Answer: d
 

Very Short Answer Type Questions

Question: To which part of the electromagnetic spectrum does a wave of frequency 5 × 10¹⁹ Hzbelong?
Answer: X-rays.

Question: Welders wear special goggles or face masks with glass windows to protect their eyes from electromagnetic radiations. Name the radiations and write the range of their frequency.
Answer: Ultraviolet radiations produced during welding are harmful to eyes. Special goggles or face masks are used to protect eyes from UV radiations. UV radiations have a range of frequency between 10¹⁴ Hz – 10¹⁶ Hz.

Question: How are radio waves produced?
Answer: Radio waves are the electromagnetic waves of frequency ranging from 500 KHz to about 1000 MHz. These waves are produced by oscillating electric circuits having inductor and capacitor.

Question: Write two uses of microwaves.
Answer: Uses of microwaves :
(i) In long distance communication
(ii) In radar systems used in aircraft navigation

Short Answer Type Questions

Question: (a) Optical and radio telescopes are built on the ground but X-ray astronomy is possible only from satellites orbiting the earth. Why?
(b) The small ozone layer on top of the stratosphere is crucial for human survival. Why?
Answer: (a) The earth’s atmosphere is transparent to visible light and radiowaves but it absorbs X-rays. X-ray astronomy is possible only from satellites orbiting the earth. These satellites orbit at a height of 36,000 km, where the atmosphere is very thin and X-rays are not absorbed.
(b) Ozone layer absorbs ultraviolet radiation from the sun and prevents it from reaching the earth and causing damage to life.

Question: (i) Which segment of electromagnetic waves has highest frequency? How are these waves produced? Give one use of these waves.
(ii) Which e.m. waves lie near the high frequency end of visible part of e.m. spectrum? Give its one use. In what way this component of light has harmful effects on humans?
Answer: (i) Gamma rays has the highest frequency in the electromagnetic waves. These rays are of the nuclear origin and are produced in the disintegration of radioactive atomic nuclei and in the decay of certain subatomic particles.
They are used in the treatment of cancer and tumours.
(ii) Ultraviolet rays lie near the high-frequency end of visible part of e.m. spectrum. These rays are used to preserve food stuff. The harmful effect from exposure to ultraviolet (UV) radiation can be life threatening, and include premature aging of the skin, suppression of the immune systems, damage to the eyes and skin cancer.

Question: Name the types of em radiations which
(i) are used in destroying cancer cells,
(ii) cause tanning of the skin and (iii) maintain the earth’s warmth.
Write briefly a method of producing any one of these waves.
Answer: (i) Gamma rays
(ii) UV rays
(iii) Infra-red radiations
Infra-red waves are produced by hot bodies and molecules. 
Infra-red waves are referred to as heat waves, because water molecules present in most materials readily absorb infra-red waves (many other molecules, for example, CO₂,NH₃ also absorb infra-red waves). After absorption, their thermal motion increases, that is they heat up and heat their surroundings.

Question: Name the constituent radiation of electromagnetic spectrum which is used for
(i) aircraft navigation.
(ii) studying crystal structure.
Write the frequency range for each.
Answer: (i) Microwaves are used in radar system for aircraft navigation. The frequency range is 3 × 10⁸ Hz to 3 × 10¹¹ Hz.
(ii) X-rays are used for studying crystals structure of solids.
Their frequency range is 3 × 10¹⁶ Hz to 3 × 10²¹ Hz. 

Question: Name the parts of the electromagnetic spectrum which is 
(a) suitable for radar systems used in aircraft navigation.
(b) used to treat muscular strain.
(c) used as a diagnostic tool in medicine.
Write in brief, how these waves can be produced.
Answer: (a) Microwaves are suitable for radar systems used in aircraft navigation.
These waves are produced by special vacuum tubes, namely klystrons, magnetrons and Gunn diodes.
(b) Infra-red waves are used to treat muscular pain. These waves are produced by hot bodies and molecules.
(c) X-rays are used as a diagnostic tool in medicine. These are produced when high energy electrons are stopped suddenly on a metal of high atomic number.

Long Answer Type Questions 

Question: State clearly how a microwave oven works to heat up a food item containing water molecules.
Why are microwaves found useful for the radar systems in aircraft navigation?
Answer: In microwave oven, the frequency of the microwaves is selected to match the resonant frequency of water molecules so that energy from the waves get transferred efficiently to the kinetic energy of the molecules. This kinetic energy raises the temperature of any food containing water.
Microwaves are short wavelength radio waves, with frequency of order of GHz. Due to short wavelength, they have high penetrating power with respect to atmosphere and less diffraction in the atmospheric layers. So these waves are suitable for the radar systems used in aircraft navigation.

Case Based MCQs

Directions of Electromagnetic Waves 
In an electromagnetic wave both the electric and magnetic fields are perpendicular to the direction of propagation, that is why electromagnetic waves are transverse in nature. Electromagnetic
waves carry energy as they travel through space and this energy is shared equally by the electric and magnetic fields. Energy density of an electromagnetic waves is the energy in unit volume of the space through which the wave travels.

Question: The electromagnetic waves propagated perpendicular to both ^→Ε.and ^→B. The electromagnetic waves travel in the direction of
(a) ^→E ^→B
(b) ^→E ×^→ B
(c)^→B^→E
(d)^→B ×^→E 
Answer: b

Question: Fundamental particle in an electromagnetic wave is
(a) photon
(b) electron
(c) phonon
(d) proton 
Answer: a

Question: The electromagnetic waves propagated perpendicular to both ^→Ε.and ^→B. The electromagnetic waves travel in the direction of
(a) ^→E ^→B
(b) ^→E ×^→ B
(c)^→B^→E
(d)^→B ×^→E
Answer: b

Question:The electric and magnetic fields of an electromagnetic waves are
(a) in opposite phase and perpendicular to each other
(b) in opposite phase and parallel to each other
(c) in phase and perpendicular to each other
(d) in phase and parallel to each other.
Answer: c

Question: For a wave propagating in a medium, identify the property that is independent of the others.
(a) velocity
(b) wavelength
(c) frequency
(d) all these depend on each other 
Answer: c