Herz and other scientists proved that is possible to produce electromagnetic waves of wavelengths ranging from several kilometers to 6×10‾³ m. The visible light is found to be electromagnetic waves of very short wave length produced by atom s and molecules,which behave as natural oscillators.By the end of last century,electromagnetic radiation ranging from several kilometers to 6 mm and from 10¯6 m ( infra – red ) to 10¯8 ( ultra violet) became known.Then , X-rays were discovered ,which were found to be electromagnetic waves of even shorter wavelength. The study of radioactive phenomenon led to the discovery of λ- rays , which are electromagnetic waves of wavelength even shorter than X-rays Then, originally existing gaps in the electromagnetic spectrum got filled on the production of corresponding electromagnetic waves experimentally.
The distribution of electromagnetic waves ( according to wavelength or frequency) in the form of distinct groups having widely differing properties is called electromagnetic spectrum.
Main parts of electromagnetic spectrum . The electromagnetic spectrum may be divided into the following main parts:
- 1.λ–rays.These waves of nuclear origin and range from 3χ10¹9 to 5×10²º Hz ,λ rays are highly energetic radiation and are mainly emitted by radioactive substances.
- X- rays . X rays were discovered by German physicist W. Rontgen . These rays are of atomic origin and are produced, when a target of an element having high atomic number is bombarded by fast moving electrons. X-rays have frequencies in the range 10 ¹6 to 3× 10¹9Hz. X-rays possess a high penetrating power.
- Ultra-voilet rays.Ultra-voilet rays where discovered by Ritter in 1801.The ultra-voilet rays are part of solar spectrum.They can be produced by the arcs of mercury and iron.They can also be obtained by passing discharge through hydrogen and xenon.The frequency of ultra-voilet rays lies in the range 8*10¹⁴ to 8*100000 Hz.
- Visible light.It forms a very narrow part of the electromagnetic spectrum and its frequency ranges from 4*10¹⁴ to 8*10¹⁴ Hz.The visible light is emitted due to the atomic excitation.Human eye is sensitive to only visible part of the electromagnetic spectrum.
- Infra-red rays.Infra-red were discovered by Herschel.Infra-red rays are heat radiation and therefore all hot bodies are the sources of infra-red rays.About 60% of the solar radiation is infra-red in nature.The frequency range of infra-red rays in 3*10¹¹ to 4*10¹⁴ Hz.
- Radiowaves.Like microwaves,radiowaves are also produced by oscillating electronic circuits The frequency of radiowaves varies from 3*10³ to 3*10¹¹ Hz.The radiowaves are used as carrier waves in radio nrobroadcastiand TV transmission.
- Electromagnetic waves are classified according to the type of the excitation.Overlapping in certain parts of the spectrum means that the corresponding wave-lengths can be produced by two methods.For example,10-⁴ m waves can be produced both with the help of an artificial oscillator and in process of thermal radiation.It may be pointed out that the physical properties of electromagnetic waves are determined by the wavelength and not by the method of their excitation.
RADIOWAVES and MICROWAVES
Radiowaves are produced by oscillating electric circuits having an inductor and a capacitor.Like radiowaves,microwaves are also produced by such electric currents.An oscillating circuit produces time varying magnetic field,which then produces time varying electric field.The two time varying fields constitute radiowavesRwhich are basically electromagnetic waves.The microwaves have completely smaller wavelength range as compared to the radiowaves.
Properties.1.Radiowaves and microwaves are electromagnetic waves and travel with speed of 3*100000000 m s-¹.
2.Radiowaves obey laws of reflection and refraction.
3.Radiowaves get diffracted from obstacles coming in their path.Since wavelength of radiowaves is quite large,the size of such obstacles should also be large.
4.Microwaves produce heat,when absorbed by matter.
APPLICATIONS.1.Radiowaves are used for wireless communication purposes.They are used to transmit radio and TV signals.
2.Radiowaves are used in radioastronomy.
3.Microwaves are used in radar systems.
4.Microwaves are used in long distance telephone communication systems.
5.Microwaves ovens are used for cooking.
6.Microwaves are used in the study of atomic and molecular structure.
Infra-red rays were discovered by Herschell.Infra-red rays are heat radiation and therefore all hot bodies are the sources of infra-red rays.About 60% of the solar radiation is infra-red in nature.The following are a few sources to produce infra-red rays:
1.Nernst lamp.The filament of Nernst lamp is made from the mixture of zirconium,thorium and cesium.When a current is passed through such a filament,then at a temperature of about 1200 K , infra-red are emitted.
Globar.It is basically a rod of silicon carbide,when heated to a temperature of about 900 K by passing current current,produces infra-red rays.