Electromagnetic Waves

Electromagnetic Spectrum

When Maxwell predicted the existence of electromagnetic waves, the visible light waves were the only ones familiar to us. People barely knew about ultraviolet and infrared rays. However, by the end of the nineteenth century, X-rays and gamma-rays were also discovered. Today, we know that electromagnetic waves include different types of waves. Electromagnetic Spectrum is the classification of these waves according to their frequency.

Remember: There is no demarked division between the waves. They are classified based on how they are produced and/or detected.

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Electromagnetic Spectrum

Here is a quick look at the electromagnetic spectrum with common names for various regions.

electromagnetic spectrum

Let’s look at each of these electromagnetic waves in the order of decreasing wavelengths.

Waves in the Electromagnetic Spectrum

Waves in the electromagnetic spectrum are broadly classified as follows:

  1. Radio waves
  2. Microwaves
  3. Infrared rays
  4. Visible rays
  5. Ultraviolet rays
  6. X-rays
  7. Gamma rays

Radio Waves

  • Radio waves are usually in the frequency range from 500 kHz to 1000 MHz.
  • Also, the range of the AM (amplitude modulated) band is between 530 kHz and 1710 kHz.
  • Further, shortwave bands use higher frequencies of up to 54 MHz.
  • TV waves range from 54 MHz to 890 MHz.
  • The FM (frequency modulated) radio band is from 88 MHz to 108 MHz.
  • Cellular phones also use radio waves to transmit voice communication in an ultra-high frequency (UHF) band.

Generation of Radio Waves

The accelerated motion of charges in conducting wires generates Radio waves. Radio and television communication systems widely use these waves.

Microwaves

  • Microwaves are short-wavelength radio waves with frequencies in the Gigahertz (GHz) range
  • Best suited for the radar systems in aircraft navigation
  • Another use of Radars is as speed-guns. These speed guns help time fastballs, tennis serves and automobiles.
  • These waves form the basis of microwave ovens. In microwave ovens, the frequency of the microwaves is selected to match the resonant frequency of water molecules. This results in a direct transfer of energy from the waves to the kinetic energy of the water molecules raising the temperature of any food containing water.

Generation of Microwaves

Special vacuum tubes called klystrons, magnetrons and Gunn diodes generate microwaves.

Infrared Rays

  • ‘Heat Waves’ is another name for Infrared rays.
  • Water molecules present in most materials readily absorb these rays.
  • After absorption, their thermal motion increases which increases their heat and that of their surroundings.
  • Many physical therapy treatments use Infrared lamps.
  • These rays also play an important role in maintaining the earth’s average temperature through the greenhouse effect.
    • Greenhouse effect: The earth’s surface absorbs the incoming visible light. Then, it re-radiates it as infrared radiations. The greenhouse gases like carbon dioxide and water vapour trap these radiations.
  • Earth Satellites deploy Infrared detectors for military purposes and to observe the growth of crops.
  • Remote switches of household appliances like TV, video recorders, etc. use infrared rays.

Generation of Infrared Rays

Hot bodies and molecules generate Infrared rays. Also, the band lies next to the low-frequency or long-wavelength end of the electromagnetic spectrum.

Visible Rays

  • Visible rays are the most familiar form of electromagnetic waves.
  • Most importantly, it is that part of the electromagnetic spectrum that is detected by the human eye.
  • Frequency range is between 4 x 1014 Hz and 7 x 1014
  • Wavelength range is from 700-400 nm.

Ultraviolet Rays

  • Ultraviolet rays have wavelengths ranging from 4 x 10-7 m (400 nm) to 6 x 10-10 m (0.6 nm).
  • These rays can have harmful effects on humans if exposed to in large quantities.
  • Ordinary glass absorbs UV rays. In other words, sit behind a glass window and avoid suntans and sunburns.
  • Welding arcs produce a large number of UV rays. Hence, welders wear special goggles or masks with glass to protect their eyes.
  • Now, UV rays have shorter wavelengths. Hence, they are focused into very narrow beams and used in high-precision applications like LASIK eye surgery.
  • Many water purifiers use UV lamps to kill germs in water.

Generation of Ultraviolet Rays

Special lamps and very hot bodies generate Ultraviolet rays. Also, the sun is an important source of ultraviolet rays.

X-rays

  • In the electromagnetic spectrum, X-rays lie beyond the ultraviolet region.
  • X-rays have wavelengths ranging from about 10-8 m or 10 nm to 10-13 m or 10-4
  • X-rays are particularly well known due to their use as a diagnostic tool in medicine.
  • Also, the treatment for certain types of cancer involves the use of X-rays.
  • X-rays can damage or destroy living tissues. Hence, you must take care and avoid unnecessary over-exposure to these rays.

Generation of X-rays

X-rays are commonly generated by bombarding a metal target with high energy electrons.

Gamma Rays

  • Gamma rays lie in the upper-frequency region of the electromagnetic spectrum
  • The wavelengths of these waves range from about 10-10 m to less than 10-14
  • An important application of Gamma rays is their extensive use in medicine to destroy cancer cells

Generation of Gamma Rays

Gamma rays are produced in nuclear reactions. Some radioactive nuclei also emit gamma rays.

Solved Examples for You

Question: List down the different types of waves in the electromagnetic spectrum. Also, specify the method of generation of each of these waves.

Solution: The waves in the electromagnetic spectrum can be broadly classified as:

  1. Radio waves – produced by the accelerated motion of charges in conducting wires
  2. Microwaves – produced by special vacuum tubes called klystrons, magnetrons and Gunn diodes.
  3. Infrared rays – produced by hot bodies and molecules.
  4. Visible rays – visible light emitted or reflected by objects
  5. Ultraviolet rays – produced by special lamps and very hot bodies, like the sun.
  6. X-rays – produced by bombarding a metal target with high energy electrons.
  7. Gamma rays – produced in nuclear reactions and also emitted by radioactive nuclei.
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