Electromagnetic Spectrum

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12th physics. NCERT notes

After the demonstration of electromagnetic waves by Hertz, electromagnetic waves in different regions of wavelength were produced by different ways of excitation.

The orderly distribution of electromagnetic waves according to their wavelength or frequency is called the electromagnetic spectrum.

Electromagnetic spectrum covers a wide range of wavelengths (or) frequencies. The whole electromagnetic spectrum has been classified into different parts and subparts, in order of increasing wavelength and type of excitation. All electromagnetic waves travel with the velocity of light. The physical properties of electromagnetic waves are determined by their wavelength and not by their method of excitation. The overlapping in certain parts of the spectrum shows that the particular wave can be produced by different methods.

In table shows various regions of the electromagnetic spectrum with the source, wavelength and frequency ranges of different electromagnetic waves.

The following are some of the uses of electromagnetic waves:

  • Radio waves: These waves are used in radio and television communication systems. AM band is from 530 kHz to 1710 kHz. Higher frequencies up to 54 MHz are used for short waves bands.
  • Television waves range from 54 MHz to 890 MHz. FM band is from 88 MHz to 108 MHz. Cellular phones use radio waves in ultra-high frequency (UHF) band.

Microwaves:

Due to their short wavelengths, they are used in radar communication system. Microwave ovens are an interesting domestic application of these waves.

Infrared waves:

  • Infrared lamps are used in physiotherapy.
  • Infrared photographs are used in weather forecasting.
  • As infrared radiations are not absorbed by air, thick fog, mist etc, they are used to take the photograph of long distance objects.
  • The infrared absorption spectrum is used to study the molecular structure.
  • Visible light: Visible light emitted or reflected from objects around us provides information about the world. The wavelength range of visible light is 4000 Å to 8000 Å.

Ultra−violet radiations

  • They are used to destroy the bacteria and for sterilizing surgical instruments.
  • These radiations are used in the detection of forged documents, fingerprints in forensic laboratories.
  • They are used to preserve the food items.
  • They help to find the structure of atoms.

X rays:

  • X rays are used as a diagnostic tool in medicine.
  • It is used to study the crystal structure in solids.

γ−rays:

Study of γ rays gives useful information about the nuclear structure and it is used for the treatment of cancer.

Table: Different types of Electromagnetic waves

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