# Important Questions for Class 12 Physics Chapter 8 Electromagnetic Waves

Home » CBSE Class 12 Physics » Important Questions for Class 12 Physics » Important Questions for Class 12 Physics Chapter 8 Electromagnetic Waves

Here we are providing important questions for class 12 Physics for Chapter Electromagnetic Waves. This includes Fill in the blank type, Very short answer type questions and Short answer type questions. Link for MCQs, Assertion Reason and Case Study or passage type questions are also provided. So if you are looking for a complete package, then you have come to the right place.

Important Questions for Class 12 Physics Chapter 8 Electromagnetic Waves

### Fill in the Blanks

(1) _____ particles radiate electromagnetic waves.

(2) Ozone layer in the atmosphere plays a protective role, and hence its depletion by _____ gas is a matter of international concern.

(3) In case of electromagnetic wave, the vibrating electric field vector ( E ) and magnetic field vector ( B ) are mutually perpendicular to each other and both are perpendicular to the direction of _____.

(4) The orderly distribution of electromagnetic radiations according to their frequency or wavelength is called _____.

(5) Electromagnetic wave is _____ in nature as the electric and magnetic fields are perpendicular to each other and to the direction of propagation of the wave.

(6) The shortest wavelength radio waves are called _____.

(7) Electromagnetic waves are not _____ by electric and magnetic waves.

(8) The _____ of electromagnetic waves does not change when it goes from one medium to another but its wavelength changes.

Q.1. Do electromagnetic waves carry energy and momentum?

Answer Answer: Yes, EM waves carry energy E and momentum p. As electromagnetic waves contain both electric and magnetic fields, there is a non-zero energy density associated with it.

Q.2. What are the directions of electric and magnetic field vectors relative to each other and relative to the direction of propagation of electromagnetic waves?

Answer Answer: Both electric field and magnetic fields are electromagnetic waves. These waves are perpendicular to each other and perpendicular to the direction of propagation.

Q.3. How is the speed of EM-waves in vacuum determined by the electric and magnetic fields?

Answer Answer: Speed of EM waves is determined by the ratio of the peak values of electric field vector and magnetic field vector. i.e.,
c=E/B

Q.4. Name the physical quantity which remains same for microwaves of wavelength 1 mm and UV radiations of 1600 Å in vacuum.

Answer Answer: Velocity (c = 3×108 m/s) This is because both are electromagnetic waves.

Q.5. A plane electromagnetic wave travels in vacuum along Z-direction. What can you say about the direction of electric and magnetic field vectors ?

Magnetic field vector along Y-axis.

Q.6. Write the expression for speed of electromagnetic waves in a medium of electrical permittivity ε and magnetic permeability μ.

Answer Answer: The speed of electromagnetic waves in a material medium in given by, v=1/√με

Q.7. The speed of an electromagnetic wave in a material medium is given by, v=1/√με, μ being the permeability of the medium and ε its permittivity. How does its frequency change?

Answer Answer: The frequency of electromagnetic waves does not change while travelling through a medium.

Q.8. To which part of the electromagnetic spectrum does a wave of frequency 5 × 1019 Hz belong?

Q.9. To which part of the electromagnetic spectrum does a wave of frequency 3 × 1013 Hz belong?

Q.10. Arrange the following electromagnetic waves in order of increasing frequency:
c -rays, microwaves, infrared rays and ultraviolet rays.

Q.11. Arrange the following electromagnetic waves in decreasing order of wavelength:
c -rays, infrared rays, X-rays and microwaves.

Q.12. Which part of the electromagnetic spectrum is absorbed from sunlight by ozone layer?

Q.13. 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.

Q.14. 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 Answer: Infrared waves are produced by hot bodies and molecules, so are referred to as heat waves.
(i) Electromagnetic wave having short wavelength than infrared waves are visible, UV, X-rays and γ-rays.
(ii) Electromagnetic wave having longer wavelength than infrared waves are microwaves, radio waves.

Q.15. Name the electromagnetic waves, which (i) maintain the Earth’s warmth and (ii) are used in

(ii) Microwaves

Q.16. How are X-rays produced?

Answer 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.

Q.17. Why are microwaves considered suitable for radar systems used in aircraft navigation?

Q.18. Which part of the electromagnetic spectrum is used in operating a RADAR?

Answer Answer: Microwaves with frequency range between 1010 to 1012 Hz are used in operating a RADAR.

Q.1. Explain briefly how electromagnetic waves are produced by an oscillating charge. How is the frequency of EM waves produced related to that of the oscillating charge?

Answer Answer: An oscillating or accelerated charge is supposed to be source of an electromagnetic wave. An oscillating charge produces an oscillating electric field in space which further produces an oscillating magnetic field which in turn is a source of electric field. These oscillating electric and magnetic field, hence, keep on regenerating each other and an electromagnetic wave is produced The frequency of EM wave = Frequency of oscillating charge.

Q.2. Identify the electromagnetic waves whose wavelengths vary as
(a) 10–12m < λ < 10–8 m (b) 10–3 m < λ < 10–1 m
Write one use for each.

Answer Answer: (a) X-rays: Used as a diagnostic tool in medicine and as a treatment for certain forms of cancer.

Q.3. State two properties of electromagnetic waves. How can we show that EM waves carry
momentum?

(i) Transverse nature
(ii) Does not get deflected by electric or magnetic fields
(iii) Same speed in vacuum for all waves
(iv) No material medium required for propagation
(v) They get refracted, diffracted and polarised
Electric charges present on a plane, kept normal to the direction of propagation of an EM wave can be set and sustained in motion by the electric and magnetic field of the electromagnetic wave. The charges thus acquire energy and momentum from the waves.

Q.4. (a) An EM wave is travelling in a medium with a velocity vector v = v î. Draw a sketch showing the propagation of the EM wave, indicating the direction of the oscillating electric and magnetic fields.
(b) How are the magnitudes of the electric and magnetic fields related to the velocity of the EM
wave?

Q.5. Name the part of the electromagnetic spectrum whose wavelength lies in the range 10–10 m. Give its one use.

Answer Answer: The electromagnetic waves having wavelength 10–10 m are X-rays. X-rays are used to study crystal structure.

Q.6. (a) How does oscillating charge produce electromagnetic waves?
(b) Sketch a schematic diagram depicting oscillating electric and magnetic fields of an em wave
propagating along + z-direction.

Answer Answer: (a) An oscillating charge produces an oscillating electric field in space, which produces an oscillating magnetic field. The oscillating electric and magnetic fields regenerate each other, and this results in the production of em waves in space.
(b) Electric field is along x-axis and magnetic field is along y-axis.

Q.7. (i) How are infrared waves produced? Write their one important use.
(ii) The thin ozone layer on top of the stratosphere is crucial for human survival. Why?

Answer Answer: (i) Infrared waves are produced by hot bodies and molecules. Important use:
(a) To treat muscular strains (b) To reveal the secret writings on the ancient walls (c) For producing dehydrated fruits (d) Solar heater (e) Solar cooker (Any one) (ii) Ozone layer protects us from harmful UV rays.

Q.8. (i) Which segment of electromagnetic waves has highest frequency? How are these waves
produced? Give one use of these waves.
(ii) Which EM waves lie near the high frequency end of visible part of EM spectrum? Give its one
use. In what way this component of light has harmful effects on humans?

Answer Answer: (i) Gamma rays have the highest frequency. These are produced during nuclear reactions and also emitted by radioactive nuclei. They are used in medicine to destroy cancer cells.
(ii) Ultraviolet rays lie near the high frequency end of visible part of EM spectrum. They are used to sterlise drinking water and surgical instruments. Exposure to UV radiation induces the production of more melanin, causing tanning of the skin.

## 1 thought on “Important Questions for Class 12 Physics Chapter 8 Electromagnetic Waves”

1. Can u give questions on circular motion

NEET Chapter-wise Test Series for PCB

CBSE Term 2 Test Series for Class 9 to 12