Notes Physics

Electromagnetic Waves


  • Definition and Concept
  • Types of Radiation
  • Detectors
  • Uses


Electromagnetic waves are produced by electromagnetic vibrations. Electromagnetic waves have electrical origin and the ability to travel in vacuum.  So, electromagnetic waves are regarded as a combination of traveling electric and magnetic forces which vary in value and are directed at right angles to each other and to the direction of travel.  In other words, they are transverse waves.


The electromagnetic waves consist of the following:

  1. Radio waves with wavelength 10-3m to 1000m.
  2. Infra-red waves with average wavelengths of 10-6m.
  3. Visible spectrum, known as light waves, with wavelengths of 7 x 10-7 m for red rays.
  4. Ultraviolet rays with wavelength of 10-8m
  5. X- rays with wavelength of 10-10 m.
  6. Gamma –rays with wavelength of 10-11m.

Radio waves:  Radio waves have the longest wavelengths.  Radio waves are emitted from transmitters and carry radio signals to radio sets.  The shortest radio waves are called microwaves.  Microwaves are used in radar and in heating hence they are used in cooking.

Infra-red waves Infra-red waves are found just beyond the red end of the visible spectrum.  They are present in the radiation from the sun or from the filament of an electric lamp. Many manufacturing industries used infra-red lamps to dry paints on painted items. They are also used for the treatment of muscular problems.


The visible spectrum is made up of red, orange, yellow, green, blue, indigo and violet rays. These are all colours of the rainbow.  When these rays combine, they form a white light.  In the visible spectrum, red rays have the longest wavelengths while the violet rays have the shortest wavelengths. The main source of light is the sun.


Ultra violet rays are located just beyond the violet end of the visible spectrum. Ultraviolet rays can be produced by quarts, mercury filaments, or the sun. Ultraviolet rays can cause certain materials to fluoresce (i,e glow).


X-rays are produced when fast moving electrons strike a metal target, which reduces their velocity.X- rays are used in hospitals to destroy malignment growth in the body and to produce X–ray photographs which can locate broken bones,  Much of X-ray in the body is harmful and can lead to sterility and adverse change in the blood. X-rays are used in industries to locate cracks in metal castings and flows in pipes.

X-rays ionize gases and have a penetrating effect such that they pass through substances opaque to white light are diffracted by crystals and unaffected by either electric or magnetic fields. 


Gamma – rays are emitted by radioactive substances such as cobalt. 60, uranium and polonium. Like x- rays, gamma rays ionize gases and darken photographic plates.  Because of their shorter wavelengths gamma rays have a greater penetrating power.


1.  Give three similarities of electromagnetic waves.  Mention two distinguishing properties of infra-red and ultraviolet rays.

2.         Mention and describe two important uses of x-rays.


The detectors of the various radiations in the electromagnetic spectrum are

i.          Gamma rays     –           Geiger-Muller tube

ii.          X- rays             –           Photographic films

iii.         Ultraviolet rays-            Photographic films, fluorescent substances

iv.         Visible rays       –           Eye, photographic film, photo electric cell

v.         Infra-red rays   –           Skin, thermometer, photo transistor, photographic film.

vi.         Radio waves     –           Radio set, Television set, Aerials


  1. How can you detect the following radiations? (i) x- rays, (ii) Visible rays, (iii) Infra-red rays.
  2. Electromagnetic waves are regarded as transverse waves, why?


  1. A knowledge of infra-red rays is used in developing infer red telescopes, infra-red signaling lamps which are useful to soldiers fighting in darkness.
  2. With the aid of photographic film which are sensitive to infra red, it is possible to take clear photographs through mist and haze.
  3. X-rays are useful in hospitals (e.g to inspect broken bones), industry (to inspect metal castings), and in science to study crystal structure of matters.
  4. Gamma rays are used to kill cancer cells in patient’s body as well as bacteria in foods and hospitals equipment.
  5. A knowledge of ultraviolet rays is used in developing ultraviolet lamps, the lamps are useful in conducting experiments on photo-electric effect.
  6. Radio waves are very important for effective communication especially when radio set, television set, walkie-talkie are involved.


  1. What is the relationship between the direction of a radio wave and the direction of its electric and magnetic fields?
  2. A radio station transmits at a frequency of 1200KHZ.  What is the wavelength of the radio wave?(c = 3.0 x 108 ms-1).


New School Physics for Senior Secondary School(M. WAnyakohaPages 364- 369)


  1. When a mass is hung on a spring, the spring stretches 6cm. Determine its period of vibration, if its pulled down and released [Π = 3.142 , g = 10m/s2]
  2. A simple pendulum has a period of 17.0s. When the length is shortened by 1.5cm its period is 8.5s. Calculate the original length.


1.         Arrange the following electromagnetic waves in their increasing order of wavelength, Infra-red rays, visible rays, radio waves and x-rays.(a) Radio waves, infra- red ray, visible rays, x-rays (b)  x-rays, visible rays, infra –red rays, radio waves (c) Visible rays, x –rays, radio waves, infra-red rays. (d) Infra-red rays, Radio waves, X- rays, Visible rays.

2.         Which of the following radiation is of nuclear origin?(a)  X- rays      (b) Visible –rays   (c ) Radio waves    (d) Gamma rays.

3.         Which of the following radiations is found useful by soldiers fighting in darkness.(a)  Gamma-rays      (b) x- rays      ( c) infra- red rays   (d ) ultra violet rays.

4.         The velocity of light in vacuum is

            (a) 3. 0 x 106 m/s         (b) 3.0 x 107 m/s          (c) 3.0 x 108 m/s    (d)  3.0 x 109 m/s

5.         Which of the following has the shortest wavelength?

            (a ) Radio waves          (b) x –rays                   ( c) Infra –red rays  (d) microwaves.


1.         (a) What rays make up the radio waves?

(b) Arrange these radiations in their increasing order of wavelength.Ultraviolet rays,Radio waves Visible spectrum, Infra-red wavesX- rays Gamma –rays

2.         (a) What is a radar?

(b) What type of electromagnetic radiation does it use?

(c) How does it function?

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