# Class 12 Physics Chapter 8 Important Questions Electromagnetic Waves

Q 1:- What are electromagnetic waves? Ans:- Electromagnetic waves are waves propagating in free space (vacuum), without any material medium, caused by

Science is a complex and challenging subject, as it involves so many principles and concepts that are difficult to memorize. Those student who opt for science have to face many challenges and work hard to get good marks in the exam. In this lesson, students will learn about Electromagnetic Waves. The best solution of the problem is to practice as many Physics Class 12 Chapter 8 Important Questions as possible to clear the doubts.

Q 1:- What are electromagnetic waves?
Ans:- Electromagnetic waves are waves propagating in free space (vacuum), without any material medium, caused by coupled time-varying (oscillating) electric and magnetic fields.

Q 2:- What is the nature of electromagnetic waves?
Ans:- The electromagnetic waves are transverse in nature. Directions of oscillating electric field E and magnetic field B are perpendicular to each other as well as perpendicular to the direction of propagation of electromagnetic waves.

Q 3:- How does a charge oscillating at certain frequency produce electromagnetic waves?
Ans:- The oscillating charge produces an oscillating electric field in space. It produces an oscillating magnetic field, which, in turn, is a source of oscillating electric field and so on. In this manner, the oscillating electric and magnetic fields regenerate each other and as a result, an electromagnetic wave propagates through free space.

Q 4:- What are the directions of electric and magnetic field vectors relative to each other and relative to the direction of propagation of electromagnetic waves?
Ans:- The directions of electric field E and magnetic field B are mutually perpendicular and simultaneously both of them are in perpendicular directions relative to the direction of propagation of electromagnetic waves.

Q 5:- What is displacement current?
Ans:- Displacement current is the current set up in space due to a time changing electric field or electric flux.

Mathematically, it is given by

Displacement current, I_D=\epsilon_0.\frac{d\phi_E}{dt}

Q 6:- A plan electromagnetic wave travels in vacuum along y-direction. What can you say about the direction of electric and magnetic field vectors?
Ans:- When a plane electromagnetic wave travels in vacuum along y-direction, then electric field E and magnetic field vector B are along z-direction and x-direction respectively or vice versa.

Q 7:- What feature of electromagnetic waves led Maxwell to conclude that light itself is electromagnetic wave?
Ans:- The fact that electromagnetic waves can travel through vacuum with a velocity equal to the velocity of light (c=3×108m/s) led Maxwell to conclude that light itself is electromagnetic wave.

Q 8:- How are amplitudes of oscillating electric and magnetic fields related to?
Ans:- The ratio of amplitudes of electric and magnetic fields is equal to the speed of light. Mathematically,

\frac{E_0}{B_0}=c

Q 9:- What is the phase relationship between oscillating electric and magnetic fields in an electromagnetic wave?
Ans:- The electric field E and magnetic field B in an electromagnetic wave are exactly in same phase condition.

Q 10:- Write the following radiations in ascending order in respect of their frequencies:

X-rays, microwaves, ultraviolet rays and radio waves
Ans:- The radiations arranged in ascending order in respect of their frequencies are radio waves, microwaves, ultraviolet rays and X-rays.

Q 11:- Arrange the given electromagnetic radiation in the descending order of their frequencies:

Infrared rays, X-rays, ultraviolet rays and γ-rays
Ans:- The radiations arranged in descending order in respect of their frequencies are γ-rays, X-rays, ultraviolet rays and infrared rays.

Q 12:- From the the following, identify the electromagnetic waves having the (a) maximum, and (b) minimum frequency. Radio waves, γ-rays, visible light, microwaves, UV rays and infrared rays
Ans:- Out of various types of electromagnetic waves given here:

• (a) gamma rays have maximum frequency, and
• (b) radio waves have minimum frequency.

Q 13:- Arrange the following radiations in an ascending order of their wavelengths: Infrared rays, radio waves, γ-rays and microwaves
Ans:- The radiations arranged in an ascending order of wavelengths are γ-rays, infrared rays, microwaves and radio waves.

Q 14:- Arrange the following radiations in descending order of their wavelengths:

X-rays, radio waves, blue light, infrared light
Ans:- The radiations arranged in descending order of their wavelengths are radio waves. infrared light, blue light and X-rays.

Q 15:- Name the physical quantity which remains same for microwaves of wavelength 1 mm and UV radiations of 1600 Å in vacuum.
Ans:- Both microwaves and UV radiations travel with same speed of 3×108 m/s in vacuum in spite of the difference in their wavelengths.

Q 16:- Write the frequency limits of visible light portion of electromagnetic spectrum.
Ans:- Visible light region of electromagnetic spectrum extends from about 4.3 x 1014 Hz to 7.5 x 1014 Hz.

Q 17:- Name the electromagnetic radiation to which waves of wavelength in the range of 10-2 m belong. Give one use of this part of electromagnetic spectrum.
Ans:- Microwaves have wavelength in the range of 10-2 m. These are used in RADAR systems, in microwave telecommunication and in microwave oven.

Q 18:- Name the part of the electromagnetic spectrum of wavelength 102 m and mention its one application.
Ans:- Radio waves are the electromagnetic waves of wavelength 102 m. These are used for short-band radio communication.

Q 19:- Name the electromagnetic waves that have frequencies greater than those of ultraviolet light but less than those of γ-rays.
Ans:- Frequencies of X-rays are greater than those of ultraviolet light but less than those of γ-rays

Q 20:- Which part of electromagnetic spectrum is absorbed from sunlight by ozone layer?
Ans:- Ultraviolet rays are absorbed from sunlight by ozone layer.

Q 21:- Name the electromagnetic radiations used for studying crystalline structure of solids.
Ans:- X-rays are used for studying crystalline structure of solids.

Q 22:- Which part of electromagnetic spectrum is used for LASIK eye surgery, and why?
Ans:- Ultraviolet rays are generally used for LASIK eye surgery because being of shorter wavelengths, these can be easily focussed into very narrow beam for high precision required in the surgery.

Q 23:- Identify the different types of electromagnetic radiations which are used (a) to kill germs, and (b) for physical therapy.
Ans:-
(a) Ultraviolet rays are used to kill germs in water purifiers as well as in hospitals or hotels, etc.

(b) Infrared rays are used for physical therapy so as to provide relief in muscular pains.

Q 24:- Name one method each for the: (a) production, and (b) detection of X-rays.
Ans:-
(a) One common way to produce X-rays is to bombard a metal target by high-energy electrons. This method is followed in coolidge X-ray tubes.

(b) X-rays can be detected either by a photographic film or Geiger tubes or an ionisation chamber.

Q 25:- Write one method each of:

• (a) production, and
• (b) detection of microwaves.

Ans:- (a) Microwaves can be produced by special vacuum tubes called klystrons, magnetrons and Gunn diodes.

(b) Microwaves can be easily detected by using point contact diodes.

Q 26:- Which part of electromagnetic spectrum is generally used in remote switches used in household electronic systems such as TV sets, video recorders, etc.?
Ans:- Infrared rays are used in remote switches used in household electronic systems.

Q 27:- Which part of electromagnetic spectrum is used in radar systems?

Q 28:- Can displacement current be a source of magnetic field?
Ans:- Yes, just like conduction current, the displacement current is a source of magnetic field.

Q 29:- What is the basic difference between displacement current and conduction current?
Ans:- Conduction current is due to flow of current carriers (generally, conduction electrons in metallic conductors) in the circuit. Displacement current is due to time-varying electric field and set up in space even when no conductor is present. Again, conduction current flows even for steady electric fields. But displacement current does not exist in state of steady electric field.

Q 30:- Are Maxwell's equations true for different types of media: dielectrics, conductors, plasmas, etc.?
Ans:- Yes, Maxwell's equations are true for different types of media including dielectrics, conductors, plasmas, etc.

Q 31:- What does an electromagnetic wave consist of? On what factors, does its velocity in vacuum depend?
Ans:- An electromagnetic wave consists of coupled electric and magnetic fields varying both in space and time. The two fields are perpendicular to each other as well as to the direction of propagation of electromagnetic wave.

The velocity of an electromagnetic wave in vacuum depends on electric permittivity \epsilon_0 and magnetic permeability \mu_0 of vacuum. In fact, c=\frac{1}{\sqrt{\mu_0\epsilon_0}}

Q 32:- What is the direction of propagation of an electromagnetic wave in terms of associated E and B?
Ans:- The direction of propagation of an electromagnetic wave is given by the direction of vector product (E × B).

Q 33:- Which physical quantity is same in vacuum for X-rays of wavelength 10-10 m, yellow light of wavelength 550 nm, heat radiation of wavelength 1 um and radio waves of wavelength 450 m, and why?
Ans:- All the radiations mentioned here are electromagnetic waves and all of them travel in free space with constant speed 3×108m/s irrespective of their wavelengths.

Q 34:- Name the characteristics of electromagnetic waves that (a) increases, and (b) remains constant in the electromagnetic spectrum as one moves from radio wave region towards ultraviolet region.
Ans:-
(a) As one moves from radio wave region towards ultraviolet region, the frequency of a wave goes on increasing.

(b) The speed of electromagnetic waves remains constant as one moves from radio wave region to ultraviolet region.

Q 35:- What is the range of frequencies used for TV transmission? What is common between these waves and light waves?
Ans:- The range of frequencies used for TV transmission varies from 54 MHz to 890 MHz. These waves are electromagnetic waves just like light waves and travel with same speed of 3x108 m/s in free space.

Q 36:- What is the range of frequencies used in satellite communication? What is common between these waves and light waves?
Ans:- Short radio waves (or microwaves) of frequencies ranging from 3.7 GHz to 6.5 GHz are commonly used for a satellite communication. These waves, just like light waves, are electromagnetic waves and travel with same speed of 3x108 m/s in free space.

Q 37:- Why are infrared radiations referred to as heat waves also? Name the radiations which are next these radiations in electromagnetic spectrum having (a) shorter wavelength, and (b) longer wavelength
Ans:- Infrared radiations incident on different materials are readily absorbed by water molecules present in these materials and consequently, their thermal motion increases, ie, water molecules heat up and heat their surroundings too. Due to this reason, infrared radiations are also referred to as heat waves.

• (a) Electromagnetic radiation next to infrared radiation but having shorter wavelength are visible light.
• (b) Electromagnetic radiation next to infrared radiation on longer wavelength are the microwaves.

Q 38:- Name the electromagnetic waves used for studying crystal structure of solids. What is its frequency range?
Ans:- X-rays are used for studying crystal structure of solids. Their frequency range is from 1017 Hz to 1020 Hz.

Q 39:- Name the part of electromagnetic spectrum which is suitable for (a) radar system used in aircraft navigation, and (b) treatment of cancer tumours.
Ans:-

• (a) Microwaves are suitable for being used in radar system used in aircraft navigation.
• (b) Gamma rays are used for treatment of cancer tumours.

Q 40:- State the condition under which a microwave oven heats up a food item containing water molecules most efficiently.
Ans:- For most efficient heating of a food item. containing water molecules in a microwaves oven, the frequency of microwaves should match with natural frequency of vibrations of water molecules so that due to resonance phenomenon, water molecules absorb whole energy and are easily heated up.

Q 41:- What is the ratio of speed of X-tays ana infrared rays in vacuum, and why?
Ans:- The ratio is 1, because both X-rays and infrared rays are electromagnetic waves and in vacuum, they travel with a constant speed of 3x108 m/s.

Q 42:- Which waves are used in cellular phones for voice communication?
Ans:- Radio waves of frequency 800 MHz or more are used for voice communication through cellular phones.

Q 43:- Why do welders wear special glass goggles ar facemasks with glass windows?
Ans:- Welders often wear special glass goggles or facemasks with glass windows to protect their eyes from the effect of large quantity of UV radiations being emitted by welding ares. When he uses glass goggles or facemasks, then UV radiations are absorbed by glass and his eyes are not affected.

Q 44:- Identify the following electromagnetic radiations as per the wavelengths given below. Write one application of each.

• (a) 10-3 nm
• (b) 10-3 m
• (d) 1 nm

Ans:- (a) Radiation of wavelength 10-3 nm (or 10-12 m) are γ-rays which are used in cancer therapy and to cause nuclear reactions.

(b) Radiations of wavelength 10-3 m are microwaves which are used in radar system for aircraft navigation and in microwave oven.

(c) Electromagnetic radiations of wavelength 1 nm (or 10-9 m) are X-rays which are used as a diagnostic tool in medicine and for studying crystal structure of solids.

Q 45:- Which part of the electromagnetic spectrum has the largest penetrating power?
Ans:- Gamma rays have the largest penetrating power out of different parts of the electromagnetic spectrum. It is because their frequency is maximum.

Q 46:- When can a charge act as a source of electromagnetic wave?
Ans:- A charge at rest or a steadily moving charge cannot be a source of electromagnetic wave. Only an accelerating charge can act as a source of electromagnetic wave. Since an oscillating charge is an accelerating charge, hence an oscillating charge acts as a source of electromagnetic wave.

Q 47:- Why does light travel in vaccum but sound cannot do so?
Ans:- Light waves are electromagnetic waves and, thus, they can travel in vacuum. The sound waves are mechanical waves and they need a material medium for their propagation, i.e., sound waves cannot travel through vacuum.

Q 48:- What is the frequency of electromagnetic waves produced by oscillating charge of frequency v?
Ans:- The frequency of electromagnetic waves produced by oscillatory charge of frequency v is also v.

Q 49:- Which of the following if any, can act as a source of electromagnetic waves?

• (a) a charge moving with constant velocity
• (b) a charge moving in a circular orbit
• (c) a charge at rest Give reason.

Ans:- Out of three situations, only a charge moving in a circular orbit can act as a source of electromagnetic waves. We know that only an accelerating charge can be a source of electromagnetic waves. Since a charge moving in a circular orbit is an accelerating charge, having a centripetal acceleration, it can act as a source of electromagnetic waves.

Q 50:- Name the radiations of electromagnetic spectrum which are used in:

• (a) warfare to look through fog,
• (b) radar and geostationary satellites, and
• (c) studying the structure and properties of atoms and molecules.

Ans:-

• (a) Infrared rays are used in warfare to look through fog and haze, because these can easily pass through hare and fog.
• (b) Microwaves are used in radar and geostationary statellites on account of their high frequencies (of the order of GHz) and consequent large range.
• (c) Visible light radiations are used for studying the structure and properties of atoms and molecules.

Q 51:- Name the constituent radiation of electromagnetic spectrum which:

• (a) is used in satellite communication,
• (b) is used for studying crystal structure,
• (c) is similar to the radiations emitted during decay of radioactive nuclei
• (d) has its wavelength range between 390 nm and 700 nm, (e)is absorbed from sunlight by ozone layer, and
• (f) produces intense heating effect.

Ans:-

• (a) Microwaves are used in satellite communication.
• (b) X-rays are used for studying crystal structure.
• (c) Gamma rays are similar to the radiations emitted during decay of radioactive nuclei.
• (d) Visible light radiation has its wavelength range between 390 nm and 700 nm.
• (e) Ultraviolet radiations are mainly absorbed from sunlight by ozone laver present in upper part of atmosphere.
• (f) Infrared rays produce intense heating effect when falling on a material.