Electromagnetic waves (E.M. waves)
(A)nature of E.M. waves
Electromagnetic waves are members of a family of waves which are produced by electromagnetic vibration. Electromagnetic waves are regarded as a combination of travelling electric and magnetic forces which vary in value and are directed at right angles to each other and to the direction of travel.
Electromagnetic waves are those that do not require a medium for their propagation.
(B)types of electromagnetic waves, sources and uses
Radio waves have the longest wavelengths. The wavelengths vary from a few millimetres to several kilometres. Radio waves are emitted from transmitters and carry radio signals to radio sets.
Note: the shortest waves are known as microwaves. Microwaves are used in radar, and in heating, hence , they are used in cooking.
Infrared waves (I.R 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. When a body becomes white hot, white light (i.e, all the colours of the visible spectrum) and infrared radiation are emitted. Many manufacturing industries use infrared lamps to dry paints on painted items. They are also used for treatment of muscular complaints.
Visible spectrum or light waves are made up of red, orange, yellow, green, blue, indigo, and violet-rays. These are colours of the rainbow. The main source of light is the sun, therefore when objects are heated to a very high temperature, such objects become white-hot and give out white light.
Ultra-violet rays (u.v) are located just beyond the violet end of the visible spectrum. These rays can be produced by quartz, Mercury filament, or the sun.ultraviolet rays can cause certain materials to glow; they also affect photographic plates, by reacting with the chemicals which coat the photographic plates.
Note that they cause sunburn when they fall on the human body.
X-rays have shorter wavelengths than ultraviolet rays. X-rays are produced when fast moving electrons strike a metal target, which reduces their velocity. X-rays are used in hospitals to destroy malignant growths in the body, and to produce X-rays photographs which can locate broken bones.
Note: load of X-rays in the human body is harmful and can lead to sterility and adverse change in the blood.
Gamma-rays (y) are similar to X-rays. The wavelengths of y-rays are shorter than the wavelengths of X-rays. Gamma-rays comes from atomic nuclei whereas X-rays come from energy changes in the electrons outside the nucleus. Gamma-rays have high penetrating power, the uses of gamma-rays are similar to those of X-rays.
Note that v= f∆
Where v is Velocity of wave, f = wave frequency and ∆ = wavelength.
1.Frequency is measured in? (Wassce 2018)
A. Meter per second
Solution: the SI unit for frequency is Hertz
2. Which of the following electromagnetic waves is most energetic? (Wassce 2018)
A. Radio waves
D. Ultraviolet rays
Solution: gamma-rays have high penetrating power which makes them more energetic than any other electromagnetic waves.
3. A piece of cloth appears green in sunlight. When held at a red light, it will appear? (Wassce 2001)
4. The invisible part of the spectrum of white light consists of the following colours: (Wassce 1997)
A. Red, orange and yellow only
B. Infra-red, ultra-violet and blue only
C. Indigo and green only
D. Violet and red only
E. Ultra-violet and infra-red only
5. The angle of deviation of light of various colours passing through a triangular prism increases in the order. (Jamb 2013)
A. Red > green > blue
B. Green > violet > blue
C. Blue > red > green
D. Blue > green > red
6. Heat is radiated by all hot objects in the form of ? (Jamb 2010)
A. Light energy
B. Solar energy
C. Infrared rays
Solution: infrared rays have to do radiation from the sun or from filament of an electric lamp.
7. X-rays can be used in the study of Crystal structures because they? (Jamb 2004)
A. Have an extremely short wavelength
B. Have a very long-reaching wavelength
C. Are very fast
D. Are invisible
8. The electromagnetic waves that are sensitive to temperature changes are? (Jamb 1997)
C. Ultraviolet rays
D. Infra-red rays