Important Questions Class 12 Physics Chapter 8

Important Questions for CBSE Class 12 Physics Chapter 8 – Electromagnetic Waves

These important questions for class 12 physics chapter 8 will teach students about electromagnetic waves. After reviewing these CBSE revision notes, students would understand what electromagnetic waves are. Students would also be better prepared for their exams by studying these chapter 8 class 12 physics important questions and could solve several CBSE sample papers after studying these physics class 12 chapter 8 important questions.

These notes also come with important formulas and CBSE extra questions that can be used to assess your comprehension.

CBSE Class 12 Physics Chapter 8 Important Questions

Study Important Questions for Class 12 Physics Chapter 8 – Electromagnetic Waves

Here are some important questions that students can study to better prepare them for their examinations.

Short Answer Questions (2 Marks)

  1. A capacitor’s charging current is 0.25Ω. What is the current displacement between its plates?

Ans. The difference between displacement current and charging current is 0.25 A.

  1. What are some uses of infrared radiation?

Ans. The uses of infrared radiation are as follows:

(a) Infrared light is used to snap photos in foggy conditions.

(b) Infrared radiations are also used to reveal hidden texts on antique walls.

  1. Which electromagnetic spectrum component radiation is being used?

(a) To take pictures of the human body’s internal organs; 

(b) To aid with aircraft navigation

Ans. Each component of the electromagnetic spectrum emits –

(a) X-rays are typically utilized to photograph the human body’s internal organs.

(b) Aircraft navigation employs microwaves.

  1. Would the earth’s average surface temperature be greater or lower than it is currently if there were no atmosphere?

Ans. The globe warms as a result of the greenhouse effect, which traps infrared energy inside the atmosphere. The earth’s average temperature would have been established by law as a result.

  1. Why aren’t sky waves used to broadcast TV signals? Please provide two ways to improve the TV transmission’s range.

Ans. Since the ionosphere does not reflect sky waves, they are not used to transmit television signals.

Techniques for extending TV transmission range:

(a) Tall antenna;

(b) Geostationary satellites

  1. (1) Describe the situation in which a microwave oven most effectively warms food products with water molecules.

Ans. The microwaves’ frequency must match the water molecules’ resonant frequency in the food item.

(2) List the emf radiations that are adjacent to these radiations.

(a) Shorter wavelength 

Ans. The radiation with a shorter wavelength is visible light.

(b) Longer wavelength

Ans. The radiation next to longer wavelengths is microwave light.

  1. Electromagnetic waves with a wavelength

(a) λ1 is used to treat muscular strain.

(b) λ2 is used by an FM radio station for broadcasting.

(c) λ3 is used to detect fractures in bones.

(d) λ4 is absorbed by the ozone layer of the atmosphere.

Name the region of the electromagnetic spectrum that these radiations fall within. Also, sort these wavelengths in order of decreasing magnitude.

Ans. (a) Muscular strain is treated by infrared radiation.

(b) FM broadcasting utilizes radio and microwave waves.

(c) Bone fractures can be found using X-rays.

(d) The ozone layer in the atmosphere absorbs ultraviolet radiation.

Their wavelengths are listed in decreasing order by λ2 > λ1 > λ4 > λ3.

  1. (a) Draw a graph of an em wave that is linearly polarised and moving in the Z direction, indicating the axes in which the electric and magnetic fields are oscillating.

Ans. The graph of an em wave that is linearly polarised and moving down the Z-axis.

(b) Explain how the electric and magnetic field amplitudes and the speed of light relate.

Ans. (i) The link between the electric and magnetic field amplitudes and the speed of light is C= E0B0

  1. A dc source has charged a capacitor. What are the levels of displacement and conduction currents at full charge?

Ans. The source will keep the plates’ potential constant at full charge. Conduction current and displacement current will have zero amplitudes.

  1. How do electromagnetic waves get their energy from a charge q oscillating at a given frequency?

Ans. The electric field and magnetic field that are created when the charge q moves more quickly will alter space and time, producing other fields, B and E, that will continue the E.M. pattern.

Long Answer Questions (3 Marks)

  1. What kind of electromagnetic waves has wavelengths between 10-7 and 10-9 metres? What causes these waves to occur? Note the two purposes.

Ans. (i) Ultraviolet rays are electromagnetic waves with a wavelength between 10-7 and 10-9 metres.

(ii) The sun is the main source of UV radiation. Additionally, specialized lamps and extremely heated bodies emit UV rays.


  • In LASIK eye surgery, UV rays are employed.
  • In water purifiers, UV lamps are used to kill bacteria.
  1. (i) Determine which region of the electromagnetic spectrum is:

(a) appropriate for aircraft navigation radar systems,

(b) created by firing high-speed electrons at a metal target.

(ii) Why does a galvanometer briefly deflect while a capacitor is being charged or discharged? Compose the phrase required to explain this observation.

Ans. (i) (a) Microwaves

(b) X-rays

(ii) Conduction current (ic) and displacement current (id) are added to create total current i Hence, i = ic + id = ic + 0 d0dt

This indicates that we have no displacement current (id = 0) and conduction current (ic = In) the connecting wires outside the capacitor plates. On the other hand, (ic = 0), there is no conduction current and just displacement current inside the capacitor, hence i = id. It explains why there is a brief deflection in the galvanometer when a capacitor is being charged or discharged.

  1. How can oscillating charges create em waves?

Create a drawing of em waves with linear polarisation moving in the Z direction. Indicate the electric and magnetic fields’ prevailing oscillation directions.

Ans. Charges oscillate to create em waves.

(a) An example is a charge that oscillates at the same frequency. This causes an oscillating magnetic field, which in turn creates an oscillating electric field in space, and so forth. As the waves move through space, the oscillating electric and magnetic fields thus regenerate one another. Naturally, the frequency of the charge oscillation is the same as the frequency of the electromagnetic wave.

(b) A sketch of an electromagnetic wave travelling in a plane along the z-axis with an oscillating magnetic field B and an oscillating electric field E.

A drawing of em wave

The drawing shows a plane electromagnetic wave moving in the z-direction while oscillating in the x- and y-axes, respectively, along with the electric and magnetic fields.

  1. (a) A d.c. the source is crossed by an ammeter and a capacitor linked in series. Why does the ammeter briefly deflect while the capacitor is charging? When completely charged, what would the deflection be?

(b) How is the term owing to displacement current incorporated into the generalized Ampere’s circuital law obtained?

Ans. (A) The transient current that passes through the circuit when the capacitor is charging is what causes the brief deflection. When the capacitor is completely charged, the deflection would be zero.

(B) In accordance with Ampere’s circuital equation, the magnetic field B and constant current I are connected as follows:

Maxwell demonstrated the logical inconsistency of this relationship. He explained this contradiction as follows:

For loop C1, Ampere’s circuital law results in

C1 B. d l = 0 I ……. (ii)

Loop C2 lies in the region between the plates

Therefore, c2 B. d l = 0 ……. (ii)

It is expected that, C1 B. d l = C2 B. d l …….. (iv)

This contradicts common sense. Maxwell, thus, proposed the idea of displacement current.

Therefore, displacement current is the current that is present in an area where the electric field and, consequently, the electric flux are dynamically changing.

ID= 0dEdt

Where ID is displacement current and E is electric flux,

= 0 ( I + 0 dEdt )

Nowadays, it is known as Ampere-Maxwell law. This is how Ampere’s circuital law gets generalized.

  1. Respond to the following questions:

(i) Illustrate how EM waves transfer energy and momentum using a straightforward example.

(ii) How are microwaves made? Why is it required to adjust the microwave frequency in microwave ovens so that it coincides with the resonant frequency of water molecules?

(iii) List two significant applications for infrared wavelengths.

Ans. (i) Consider a plane that is parallel to the wave’s propagation direction. The em wave’s electric and magnetic fields will cause this aircraft to charge up electrically. This illustrates how em waves carry energy and motion.

(ii) Special vacuum tubes, such as the Klystron/Magnetron/Gunn diode, are used to create microwaves. In microwave ovens, the frequency of the microwaves is chosen to coincide with the resonant frequency of water molecules, allowing for an effective transfer of energy to the molecular kinetic energy.

(iii) Significant applications for infrared waves

  • The greenhouse effect is connected to infrared waves.
  • They are applied to remote switches for home electronics.
  1. Respond to the following inquiries:

(a) Identify the em waves that are created when a nucleus decays radioactively. Also identify their frequency range.

(b) While working, welders don specialised glass goggles. Why? Explain.

(c) Why are heat waves frequently used to describe infrared waves? Submit just one application.

Ans. (a) γ-rays; Frequency range: 1018 Hz to 1022 Hz

(b) Because doing so will shield your eyes from the high UV radiation that welding produces, as well as from glare and flying sparks.

(c) Because infrared waves are em waves with a longer wavelength (less frequency) and are generated by molecules of hot things that vibrate extremely fast.


  • Utilized in the electronic system remote switches in homes.
  • Used to protect fruits that have been dried.
  • Used in solar cookers and water warmers.
  1. Respond to the following:

(a) List the em waves that are employed in the therapy of specific cancer types. Their frequency range in writing.

(b) Humans depend on the thin ozone layer that covers the stratosphere. Why?

(c) Why does the em waves’ momentum on the surface transfer seem so little?

Ans. (a) Certain types of cancer can be treated using gamma (γ) rays. They operate between 1018 and 1022 Hz in frequency.

(b) Most of the dangerous UV rays that the sun emits toward the Earth are absorbed by the tiny ozone layer atop the stratosphere. UVA, UVB, and UVC radiations are among them, which have the power to obliterate the Earth’s biosphere. This layer is, therefore, essential for human life.

(c) As a result, the small value of the Planck’s constant causes the momentum imparted by the em waves impacting the surface to be relatively minimal. According to de-relation, Broglie’s electromagnetic wave with a wavelength of 1.00 nm will provide momentum (p), for instance.

p = h = 6.63 X 10-341 X 10-9 = 6.63 X 10-25 kg ms-1

The momentum’s value is quite low.

  1. Respond to the following:

(a) Identify the em waves that are appropriate for radar systems used in aeroplane navigation. List the frequency range for these waves.

(b) Should the Earth’s average surface temperature be greater or lower if there was no atmosphere? Explain.

(c) The surface on which an em wave is impacted is under pressure. Justify.

Ans. (a) Radar systems employ microwave technology. Its range of frequencies:

1010to 1012Hz

(b) Without the earth’s atmosphere, there would be no ozone layer to shield the planet from UV rays, and the earth’s surface temperature would have been lower due to the greenhouse effect, making it more difficult for humans to survive.

(c) Because an EM wave carries energy and motion, it impacts the surface under pressure.

Em waves have a negligibly little amount of pressure on the surface they strike.

It comes as a result of the photon’s incredibly low momentum, which can be based on the de-Broglie equation: ( = hp)

Or p =h = 6.63 X 10-3410-9 = 6.63 X 10-25 kg ms-1

  1. Write the two (possible) sets of expressions for an electromagnetic wave travelling in a plane and moving down the Z-axis, along with the wave’s oscillating electric and magnetic fields. What relationship exists between the peaks of these (oscillating) fields?

Ans. We have the following for the e.m. wave propagating along the z-axis:

E = E0 sin (k +t) and B= B0 sin (k + t)

There are two types of electric and magnetic fields that could exist:

Ex= E0 sin (k – t)     By= B0 sin (k – t) and

Ey = E0 sin (k + t)   Bx= B0 sin (k + t)

The relationship between these two fields’ peak values is E0B0 = C

  1. (i) List the several forms of electromagnetic radiation.

Ans. They are employed to kill cancerous cells, produce skin tanning, and keep the earth’s temperature warm.

(ii) Briefly describe how to create any one of these waves.

  1. γ-rays
  2. ultraviolet radiation
  3. infrared rays


Method to Production

  1. By means of radioactive nuclear decay, -rays, are created.
  2. Inner shell electrons in atoms transition from one energy level to another to produce ultraviolet photons.
  3. The vibrating of atoms and molecules results in the production of infrared photons.

   Very Long Answer Questions (5 Marks)

  1. There is no current flow when a dc battery charges a perfect capacitor. On the other hand, the current constantly flows when an ac source is used. How this can be explained by using the idea of displacement current?

Ans. A dc battery allows a continuous flow of current, whereas a dc battery coupled to an ideal capacitor only offers a momentary charge.

Put, ic = dqdt

A displacement current results from a change in the electric field associated with changing the charge on capacitor plates, as seen in

id = 0 dqtdt

  1. How do electromagnetic waves get their energy from a charge q oscillating at a given frequency? Imagine an electromagnetic wave moving down the Z-axis, and draw a schematic picture showing the electric and magnetic fields.

Ans. The electric field and magnetic field that are created when the charge q moves more quickly will alter space and time, producing other fields, B and E, that will continue the E.M. pattern.

This comes from Maxwell’s view, which is backed up by

  1. Some well-known numbers related to electromagnetic radiation in various scientific contexts are listed below. Indicate which region of the electromagnetic spectrum each belongs to.

(a) In interstellar space, atomic hydrogen emits a wave at a wavelength of 21 cm (wavelength emitted by atomic hydrogen in interstellar space).

(b) 1057 MHz (Lamb shift; frequency of radiation from two nearby energy levels in hydrogen).

(c) 2.7 K [temperature linked with the isotropic radiation that permeates all space and is believed to be a remnant of the universe’s “big-bang” beginning].

(d) 5890 A0–  5896 A0 [double sodium lines]

(e) The energy of a specific transition in the 57Fe nucleus connected with a well-known high-resolution spectroscopic technique (Mossbauer spectroscopy) is 14.4 keV.

Ans. (a) Radio waves are electromagnetic waves with short wavelengths.

(b) Radio waves are at the short wavelength end of the spectrum.

(c) Temperature, T = 2.70 K

mis given by Planck’s Law as:

m= 0.292.7 = 0.11 cm

In this wavelength, microwaves emerge.

(d) This is the visible spectrum’s yellow light.

(e) The connection provides energy for transition.

E= h

Where, h = Planck’s Constant = 6.6 X 10-34 Js

= Frequency of Radiation

Energy, E = 14.4 KeV

Hence, = Eh = 14.4 X 103X 1.6 X 10-196.6 X 10-34 = 3.4 X 1018 Hz. This corresponds to X- rays.

  1. Respond to the following inquiries:

(a) Short-wave bands are used for long-distance radio broadcasts. Why?

(b) To transmit TV over great distances, satellites are required. Why?

(c) While optical and radio telescopes are constructed on the ground, X-ray astronomy can only be carried out from satellites orbiting the earth. Why?

(d) The stratospheric ozone layer, a thin layer of ozone, is essential for human survival.


(e) Would the earth’s average surface temperature be greater or lower if it didn’t have an atmosphere?

(f) According to some scientists, the earth would experience a harsh “nuclear winter” that would have a catastrophic impact on life as we know it. What might be the foundation for this forecast?

Ans. (a) Shortwave bands are used for long-distance radio transmissions because only these bands will be bent by the ionosphere.

(b) Since television signals have high frequencies and energy, satellites are required for long-distance TV transmissions. As a result, the ionosphere does not reflect these signals. Satellites are, therefore, useful for reflecting TV transmissions. Additionally, they support distant TV transmissions.

(c) In terms of X-ray astronomy, the atmosphere absorbs X-rays. However, it is permeable to visible and radio wavelengths. As a result, optical and radio telescopes are constructed on the ground, whereas X-ray astronomy can only be accomplished using satellites in Earth’s orbit.

(d) For humans to survive, the thin ozone layer at the top of the atmosphere is essential because it absorbs the sun’s harmful UV radiation and keeps it from reaching the surface of the Earth.

(e) There wouldn’t be a greenhouse effect on Earth’s surface if there weren’t an atmosphere. As a result, the Earth’s temperature would drop quickly, making it frigid and challenging for humans to survive.

(f) On Earth’s surface, a worldwide nuclear conflict would be catastrophic. After a nuclear war, the Earth will experience a harsh winter because the smoke from the conflict will cover the majority of the sky, blocking solar light from reaching the atmosphere. Additionally, it will cause the ozone layer to thin.

To help students with exam preparation, Extramarks provides Class 12 Physics Chapter 8 important questions. Extramarks also provides the best quality CBSE previous year’s question paper based on the updated NCERT books and according to the latest CBSE syllabus. The questions are well-structured and cover all the important concepts. These can be of great help in last-minute revision.

Q1-Among the following rays of the electromagnetic spectrum, which one has the shortest wavelength?



b-Ultraviolet rays

c-Infrared rays



X-rays has shortest wavelength in the electromagnetic spectrum.

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FAQs (Frequently Asked Questions)

1. The following radiations should be written in ascending sequence according to their frequencies: radio waves, microwaves, UV rays, and X-rays.

X-rays, UV rays, microwaves, and radio waves.

2. What electromagnetic waves are used to analyze the crystal structure of solids? What is the frequency spectrum?

X-rays frequency range: 1017 Hz to 1020 Hz

3. What portion of the electromagnetic spectrum does the ozone layer absorb from sunlight?

Ozone layers absorb ultraviolet radiation from sunlight.

4. How do microwaves get formed?

Klystron valves or magnetron valves generate microwaves.

5. The use of microwaves in radar systems for aeroplane navigation is thought to provide some advantages.

Microwaves are appropriate for radar systems in aeroplane navigation because of their short wavelengths.