# Worksheets Chapter 8 Electromagnetic Waves Class 12 Physics

Students should refer to Worksheets Class 12 Physics Electromagnetic Waves Chapter 8 provided below with important questions and answers. These important questions with solutions for Chapter 8 Electromagnetic Waves have been prepared by expert teachers for Class 10 Science based on the expected pattern of questions in the class 10 exams. We have provided Worksheets for Class 12 Physics for all chapters on our website. You should carefully learn all the important examinations questions provided below as they will help you to get better marks in your class tests and exams.

## Electromagnetic Waves Worksheets Class 12 Physics

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.

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

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

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

C

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

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

C

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

D

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

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

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

B

Question: If μ0 be the permeability and k0 be the dielectric constant of a medium, then its refractive index is given by IMAGE 16  C

C

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

C

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.

C

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

A

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

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

D

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)

D

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

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/

C

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.

A

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

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.

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.

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.

D

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: To which part of the electromagnetic spectrum does a wave of frequency 5 × 1019 Hzbelong?

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: Write two uses of microwaves.
(i) In long distance communication

Question: Name the constituent radiation of electromagnetic spectrum which is used for
(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: (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: Identify the part of the electromagnetic spectrum used in (i) radar and (ii) eye surgery. Write their frequency range.

Question: Prove that the average energy density of the oscillating electric field is equal to that of the oscillating magnetic field.
Answer: In an electromagnetic wave, both E and B fields vary sinusoidally in space and time. The average energy density u of an e.m. wave can be obtained by replacing E and B by their rms value

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.
(ii) UV rays
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 parts of the electromagnetic spectrum which is
(b) used to treat muscular strain.
(c) used as a diagnostic tool in medicine.
Write in brief, how these waves can be produced.
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.

Question: Suppose that the electric field part of an electromagnetic wave in vacuum is
E = (3.1N/C) cos [(1.8 rad/m) y + (5.4 ×108 rad/s)t] t
(a) What is the direction of propagation?
(b) What is the wavelength l?
(c) What is the frequency n?

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: (a) Arrange the following electromagnetic waves in the descending order of their wavelengths :
(i) Microwaves
(ii) Infra-red rays
(iv) Gamma rays
(b) Write one use each of any two of them.

Question: (a) When the oscillating electric and magnetic fields are along the x-and y-direction respectively.
(i) point out the direction of propagation of electromagnetic wave.
(ii) express the velocity of propagation in terms of the amplitudes of the oscillating electric and magnetic fields.
(b) How do you show that the e.m. wave carries energy and momentum?
Answer: (a) (i) The e.m. wave propagates along z-axis.

(ii) The speed of em-waves in vacuum determined by the electric (E0) and magnetic fields (B0)is, c =E0/B0
(b) Electromagnetic waves or photons transport energy and momentum. When an electromagnetic wave interacts with a small particle, it can exchange energy and momentum with the particle. The force exerted on the particle is equal to the momentum transferred per unit time. Optical tweezers use this
force to provide a non-invasive technique for manipulating microscopic-sized particles with light.

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.

Question: The intensity of light ranges from about 1 W /m2 for a candle to about 30 mW/m2 for a modest size laser. A particular laser produces a power 4 W in a beam 0.4 mm in diameter.
(a) What is its average intensity?
(b) Find the peak electric field of the laser light.
(c) Find the peak magnetic field of the laser light.
(d) If the laser beam is aimed upward to levitate a 20 mm diameter sphere, what is the maximum mass of this sphere. Assume the sphere is perfectly reflecting.

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) B
(b) E × B
(c)BE
(d)B ×

B

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

A

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

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.