Hello, readers ЁЯШК! Here you can learn about Electromagnetic Waves, Properties of Electromagnetic Waves and electromagnetic spectrum (radio waves, microwaves, infrared, visible, ultraviolet, X-rays, gamma rays), including elementary facts about their uses.
Electromagnetic Waves
Electromagnetic Waves is a wave which is produced by accelerated charge in which electric field and magnetic field oscillating perpendicular to each other and direction of propagation of wave.
Transverse Nature of Electromagnetic Waves
An electromagnetic wave consists of electric field vectors as well as magnetic field vectors. Both are perpendicular to each other and, at the same time, perpendicular to the direction of wave propagation. Due to this property, an electromagnetic wave is known as a transverse wave.
In the below figure, the electric field vector is represented along the y-axis, the magnetic field vector is represented along the z-axis, and electromagnetic wave propagation is represented along the x-axis.
The direction of propagation of an EM wave is in the direction of the pointing vector s, given by the below formula.
Table of Contents
Properties of Electromagnetic Waves:
(1) At any instant ratio of electric field and magnetic field may be related as c = Eo / Bo.
(2) Speed of an electromagnetic wave (c) depends on permeability and permittivity of the medium c = 1 / тИЪ(╬╝o ╤Фo)
(3) Energy density of an electromagnetic wave is equal to sum of magnetic energy and electric energy (U = UE + UB) and magnetic energy is equal to electric energy.
(4) The EM wave is transverse in nature i.e. both electric and magnetic fields are oscillating perpendicular to each other and perpendicular to the direction of wave propagation.
(5) Electromagnetic waves do not require any medium for their propagation.
(6) Electromagnetic waves are uncharged hence cannot be deflected by magnetic and electric field.
(7) Electromagnetic waves do not require any medium for their propagation and their speed in vacuum is 3 x 108 m/s.
(8) Momentum of an electromagnetic wave P = U/c
(9) Electromagnetic waves show reflection, Refraction, interference, diffraction and polarization.
(10) Electromagnetic waves carry energy and rate of flow of energy crossing a unit area is described by the Poynting vector S, where
(11) In electromagnetic wave field magnitudes of E and B vary with x and t according to the expressions
E = Emax cos(kx-╧Йt)
B = Bmax cos(kx-╧Йt)
Electromagnetic Spectrum
The arrangement of electromagnetic waves in increasing or decreasing order of wavelength or frequency is called the electromagnetic spectrum.
| Name ┬а | Frequency(Hz)/ Wavelength range | Basic source /Production | Detection | Main properties | Uses |
|---|---|---|---|---|---|
| Radiowaves | 104-108 Hz 600m-0.1m | Rapid acceleration and deaccelerations of electrons in aerials ┬а LC oscillations ┬а | Receivers aerials Diodes | Reflection, diffraction | Radio communication ┬а ┬а ┬а |
| Microwaves | 109-1012 Hz 0.1 m-1 mm | * Rotation of molecules * Klystron valve or Magnetron valve. | Point contact diodes. | Reflection, refraction, diffraction, polarization┬а┬а┬а┬а | * Radar communication. Analysis of fine details of molecular and atomic structure. *Satellite communication * Microwave oven┬а |
| Infrared | 1011– 5x1014Hz 1 mm-700nm | Vibration of atoms and molecules, Heaters ┬а | Thermopiles bolometer infrared Photographic film. | Heating effect, reflection, refraction, polarization | Useful for elucidating molecular structure. * Physiotherapy * Less scattered than visible light by atmospheric particles useful for haze photography * Remote control |
| Visible light | (4-7)x1014Hz 700 – 400 nm | Electrons in atoms emit light when they move from one energy level to a lower energy level Filament lamp, sun, flames | Human eye photocells photographic film. | Reflection, refraction, interference, diffraction, polarization, photoelectric effect etc | Help in visualization. Can cause chemical reaction ┬а ┬а ┬а ┬а |
| Ultra-Violet | 1016-1017 Hz 400 nm-1 nm | Jumping of electrons in inner shells Carbon arc, discharge tubes, sun | Photocells photographic film. | Absorbed by glass. Tanning of the human skin Ionize atoms in atmosphere resulting in the ionosphere.┬а┬а | Studying molecular structure. * Food preservation. * Sterilization of surgical instruments. * Used by insect to locate nectar. * Released in welding |
| X – rays | 106 -1019 Hz 1 nm-10-3nm | Bombarding targets with very fast electrons X тАУ ray tubes | Photographic film, Geiger tubes, ionization chamber. | Effect photographic plate, ionization of gases, photoelectric effect, more energetic than UV- rays | * Radiography- medical diagnosis. * Study of crystal structure For detecting fault, crack, flaws. Radiotherapy┬а┬а┬а |
| Gamma rays | 1018– 1022 Hz <10-3 nm | Radioactive decay of the nucleus. Cyclotron, radioactive elements | Photographic film, Geiger tubes, Ionization chamber. | Similar to X тАУ rays. High penetration power | Radiotherapy. Initiate nuclear reaction. Preservation of food. Study the structure of nuclei |
Important questions from Properties of Electromagnetic Waves
Shows that the energy density of an electric field is equal to the energy density of a magnetic field in electromagnetic waves.
Answer:
ЁЯСЙ NOTE: If in question, it is asked to prove that the average energy density of an electric field is equal to the average energy density of a magnetic field in electromagnetic waves, then use the expression for the average energy density of an electric field and the average energy density of a magnetic field.
Long distance radio broadcasts use short-wave bands. Why?
Answer : Long distance radio broadcasts use short wave bands because they can be reflected through the ionosphere (an atmosphere layer up to a height of 65-400 km) easily and can be transmitted a long distance.
It is necessary to use satellites for long distance TV transmission. Why?
Answer : TV signals are not properly reflected by the ionosphere as they have distortions in signal. Therefore, signals are made to be reflected back to earth using satellites, which maintain a quality signal and a long range.
Optical and radio telescopes are built on the ground but X-ray astronomy is possible only from satellites orbiting the earth. Why?
Answer : Visible radiation and radio-waves can penetrate the earthтАЩs atmosphere, but X-rays are absorbed by it and can not reach the earth’s surface. Therefore, X-ray astronomy is possible only from satellites orbiting the Earth outside the earth’s atmosphere.
If the earth did not have an atmosphere, would its average surface temperature be higher or lower than what, it is now?
Answer : If the Earth did not have an atmosphere, its average surface temperature would be lower than it is now. ItтАЩs temperature will decrease due to the absence of green house gases, which trap solar heat and keep the earth warm.
The small ozone layer on top of the stratosphere is crucial for human survival. Why?
Answer : The small ozone layer on top of the stratosphere is crucial for human survival because it absorbs and protects us from the sun’s UV rays reaching the earth’s surface.
Some scientists have predicted that a global nuclear war on the earth would be followed by a severe тАШnuclear winterтАЩ with a devastating effect on life on earth. What might be the basis of this prediction?
Answer : A global nuclear war on the surface of the Earth would have disastrous consequences. After the nuclear war, the Earth’s atmosphere will be covered with smoke, clouds, and dust so that solar radiation will not reach the earth’s surface. The earth will experience a severe winter.
How do you show that electromagnetic waves carry energy and momentum?
Answer: When a charge oscillates at some frequency, an electromagnetic wave is produced. The frequency of the electromagnetic wave is equal to the frequency of the oscillating charge. Here, the energy associated with the electromagnetic wave comes at the expense of the source. The sun is the ultimate source of energy on the earth’s surface, and we receive energy from it in the form of electromagnetic waves.
Energy and momentum are closely associated terms. If an electromagnetic wave of energy U strikes on a surface and gets completely absorbed, total momentum delivery to the surface is p = U/E . Hence, electromagnetic waves carry momentum too.
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