ELECTROMAGNETIC RADIATION–9th Standard Science Notes and Question Bank


Choose the most appropriate answer for the followings.

1.In vaccume, electromagnetic waves travel at the speed of ----

a) 3X108 m/s b) 3X106 m/s c) 3X10-8 m/s d) 3X1018 m/s

2. Infrared rays are used in -----

a) microwave ovens b) long distance photography

c) microscopes d) treatment of cancer.

Fill in the blanks with suitable words.

1.Electric & magnetic field in Electromagnetic wave are mutually ---- to each other.(Perpendicular)

2. The range of wavelength of visible light is ----- (400 nm to 750 nm)

Match the followings.

1.Visible light

a. J.W.Ritter (5)

2. Infrared radiation

b. Wilhelm rontegen(3)

3. X-rays

c. Henry Bequerel(4)

4. Gamma rays

d. William Herschell(2)

5. Ultravoilet rays

e. Hertz (6)

6. Radio waves

f. Newton(1)

Answer the following questions:

1.What is an electromagnetic wave?

Wave Consists of varying electric and magnetic fields in mutually perpendicular planes is called Electromagnetic wave

2.Define electromagnetic radiation?

The radiations like light, heat, X-rays, Infrared rays. Microwaves, ultravoilet rays are referred to as electromagnetic radiation.

3.Mention how the directions of electric and magnetic fields in an electromagnetic wave are related to the direction of its propagation.

The direction of wave propagation is perpendicular to the planes of electric and magnetic fields.

4. Write the characteristics of electromagnetic waves.

a.These are traverse waves.

b. They react differently with matter.

c. They travel in vaccum at speed of 3X108 m/s

d. The direction of electromagnetic wave propagation is perpendicular to the planes of electric and magnetic fields.

5. What is electromagnetic spectrum? Explain.

The classification of electromagnetic radiations in the increasing or decreasing order of frequency or wavelength is called electromagnetic spectrum . Light is not the only kind of electromagnetic radiation. Ultra violet rays, infrared rays, X rays, γ rays and radio waves are also electromagnetic radiations.We are surrounded by a sea of electromagnetic radiations. The range of wavelengths of electromagnetic waves varies from about 10–15m to about 100 km.

6.Name the components of electromagnetic spectrum in the increasing orderof frequency.

components of electromagnetic spectrum in the increasing order of frequency are Radiowaves, Microwaves, Infrared rays, Visible rays, Ultraviolet rays, X- rays, γ -rays

7.List the approximate wavelength and frequency range corresponding to each constituent radiation of the electromagnetic spectrum.


wavelength in meters

Frequency in Hertz

Energy In joules


> 1X10-1

< 3X109

< 2X10-24


1X10-3 to 1X10-1

3X109 to 3X1011

2X10-24 to 2X10-22

Infrared rays

7X10-7 to 1X10-3

3X1011 to 4X1014

2X10-22 to 3X10-19

Visible rays

4X10-7 to 7X10-7

4X1014 to 7.5X1014

3X10-19 to5X10-19

Ultraviolet rays

1X10-8 to 4X10-7

7.5X1014 to 3X1016

5X10-19 to 2X10-17

X- rays

1X10-11 to 1X10-8

3X1016 to 3X1019

2X10-24 to 1X10-22

γ -rays

< 1X10-11

> 3X1019


8.State units of wavelength & frequency.

Unit of Wavelength is metre and unit of Frequeny is hertz (Hz)

9. List the uses of Microwaves of the electromagnetic spectrum.

Microwaves find applications in Radar, satelite communication and microwave ovens. They are used for experimental purposes.

10. List the uses of Radiowaves of the electromagnetic spectrum.

a.These waves are used in radio and television hence they are called radio waves.

b.Radio waves of short wavelength are used in communication systems including satellite systems, in Radars and TV broadcasting.

c.Radio waves of longer wavelengths are used in radio broadcasting.

11.What is the wavelength range of Visible light?

400 nm to À 750 nm

12 . Who invented Infrared rays.

Infrared radiation was first detected in 1800 by W. Herschel.

13.List the uses of Infrared rays of the electromagnetic spectrum.

(i)Infrared spectrum of a compound may be used for identification and in the determination of molecular structure.

(ii)They are found to be most suitable for long distance photography.

(iii)Infrared photography is used in detecting enemy concentration, in examining old paintings and in the detection of forgery of old paintings.

(iv) Infrared search lights and telescopes were developed during the second world war.

(v)In medical field, they are useful in the diagnosis of superficial tumors, dislocations of bones and in the treatment of sprains

(vi) It stimulates blood circulation.

(vii) The remote handset of a TV uses infrared radiation to control different settings.

(viii) Infrared radiations from the sun are used in solar energy devices.

14. Who discovered Ultraviolet rays .

Ultraviolet radiation was discovered by J.W. Ritter, in 1801.

15.List the uses of Ultraviolet rays of the electromagnetic spectrum.

Applications of Ultraviolet rays are

(i)Ultraviolet radiations activate some chemical reactions.

(ii)They excite fluorescent in many substances which led to the development of fluorescent tubes.

(iii) UV-radiations are used to distinguish between real gems and artificial gems.

(iv) They are used as efficient sterilisers.

(v)They are used in the treatment of rickets, diseases of the bone, skin diseases.

(vi) UV radiations of lower frequencies are useful in the synthesis of vitamin D in our bodies.

(vii) They are useful in the operation of photoelectric alarms.

16.List the uses of Visible rays of the electromagnetic spectrum.

a.It is very difficult to imagine life and other activities in the absence of light.

b. Used for Photography. c. Used in Photosynthesis of plants.

17.Who discovered X-rays?

Wilhelm Rontegen in 1895.

18.List the uses of X- rays of the electromagnetic spectrum.

(i)Fracture of bones can easily be located by X-ray photograph.

(ii)X-rays are used to locate foreign bodies such as bullets, coins, pinsetc in human body.

(iii)X-rays are used for the treatment of cancer and some skin diseases.

(iv) They are used to detect defects like cavities in castings and cracks in weldings and in locating flaws in the parts of machines and all kinds of transport vehicles. This technique is called radiography.

(v) They are used in the study of crystal structure.

19.Who discovered Gamma rays ?

Henry Bequerel.

20. List the uses of γ –rays of the electromagnetic spectrum.

(i) Gamma rays are used in the treatment of cancer.

(ii)They act as catalyst in the manufacture of some chemicals.

(iii) They are used in γ -ray microscopes.

(iv) Gamma rays are used to produce photoelectric effect

(v)They are used in radiography.

21.How is ozone layer protecting the earth from strong ultraviolet rays.

The harmful Ultraviolet radiations are absorbed by the ozone layer surrounding the earth and hence ozone layer protecting the earth from strong ultraviolet rays.

22. List the properties of electromagnetic waves.

I. Electromagnetic waves are transverse waves.

II. Electromagnetic waves have electric and magnetic fields perpendicular to each other and perpendicular to the direction of propagation of the wave.

III. Electromagnetic waves have electric and magnetic fields which vary periodically with time.

IV. All electromagnetic waves travel with the velocity of light in vacuum. (c = 3 x 108 m/s)

V. The electric and magnetic fields of an electromagnetic wave oscillate with the same frequency.

VI. Electromagnetic waves have a wide range of frequencies and wavelengths. Different electromagnetic waves have different properties due to their different frequencies.

VII. When an electromagnetic wave passes through a medium, its velocity is slower than its velocity in vacuum. The exact value of velocity depends on the frequency of radiation and the medium's density.

VIII. Electromagnetic radiation can travel in vacuum.

IX. All electromagnetic waves may not pass through the same medium. When an electromagnetic radiation is absorbed by a medium, it interacts with the atoms or molecules of the medium to increase their vibration, thereby increasing their heat.

X. Electromagnetic waves exhibit properties like reflection, refraction, scattering, interference, diffraction and polarization.

XI. Electromagnetic radiations exhibit both wave-like and particle-like nature (dual nature).

XII. Electromagnetic wave acts like a wave when light is diffracted by a grating.

XIII. Electromagnetic wave acts like stream of particles when UV radiations strike certain metals. There is generation of electric charges.

23.Einstein’s Explanation of Photoelectric effect

I. Einstein explained the photoelectric effect by extending Planck's Quantum hypothesis. Einstein assumed that:

II. Light consists of tiny energy particles called photons.

III. Photons propagate through space and interact with matter.

IV. Photons travel in vacuum with the velocity of light.

V. Radiations are emitted and absorbed as photons.

VI. On encountering an electron, photon interacts with it and transfers its energy completely to the electron.

VII. Electron is bound to the material. A certain amount of work has to be done to release an electron from the material surface. This work done is by the incident photon.

VIII. When the frequency of incident radiation is less than the threshold frequency, the energy is insufficient to release an electron. There is no photoelectric effect.

IX. When the frequency of incident radiation is greater than the threshold frequency, a part of energy is used to liberate the electrons from the material surface. The rest of the energy is utilized in raising the kinetic energy of electrons.

Important points

1] The electromagnetic waves first detected by William Herschel

2] The electromagnetic waves first detected by J.W.Ritter

3] The electromagnetic waves first detected by Roentgen

4] The electromagnetic wave used in TV broadcasting

5] The electromagnetic wave used in Radio broadcasting

6] The electromagnetic waves useful in taking photographs in the dark, fog or haze

7] The electromagnetic waves used in a diagnostic technique called Thermography in which ‘heat picture’ of the body is used to reveal the abnormally hot or cold areas of the body.

8] The radiations which cause Suntans

9] The radiations which are not absorbed by the bones.

10] The electromagnetic wave with least wavelength

11] The high frequency electromagnetic radiation put to use in a Green house to keep the plants warm

12] The layer of the atmosphere which absorbs harmful UV radiations from the Sun.

13] Electromagnetic waves used in Radiography to detect cavities in castings or cracks in welding and to locate flaws in machine parts

14] The electromagnetic wave used to study crystal structure