ELECTROMAGNETIC WAVES ELECTROMAGNETIC WAVES EM WAVES ELECTROMAGNETIC WAVES

ELECTROMAGNETIC WAVES

ELECTROMAGNETIC WAVES EM WAVES

ELECTROMAGNETIC WAVES EM Waves Let us see what is Ø A changing electric field produces changing an a. EM wave? magnetic field and vice versa which gives rise to a transverse wave known as electromagnetic wave.

ELECTROMAGNETIC WAVES Ø The time varying electric and magnetic fields are mutually The electric vector is perpendicular to each other and also perpendicular to the direction of responsible for the optical propagation of this wave. effects of an EM wave and is called the light vector.

ELECTROMAGNETIC WAVES History of EM waves Who was the first to Ø Maxwell was the first to predict the EM wave. detect EM waves? Ø Hertz produced and detected electromagnetic waves experimentally at wavelengths of 6 m. Ø Source of EM waves is accelerated charges which radiate energy in the form of EM waves. Ø Accelerating charge produces oscillating electric and magnetic fields with regenerate each other and propagate through space as waves called EM – waves. Ø An electric charge oscillating harmonically with frequency �� produces electromagnetic waves of same frequency ��.

ELECTROMAGNETIC WAVES Ø Maxwell predicted EM – waves theoretically Hertz produced and detected EM – waves experimentally at wavelength of about 6 m. Ø J. C. Bose produces EM – waves of wavelength in the range 5 mm – 25 mm. Ø Marconi discovered that if one of the terminals is connected to an antenna and the other terminal is earthed, EM – waves could radiate up to several kilometer from which wireless communication was first established.

ELECTROMAGNETIC WAVES Properties of EM Waves Ø Speed: In free space its speed is Ø Where. What 0 = absolute are the permeability, properties of 0 = absolute permittivity, EM waves? Ø E 0 and B 0 amplitudes of electric field and magnetic field vectors. In a medium,

ELECTROMAGNETIC WAVES Energy: Ø The energy in EM waves is divided equally between electric and magnetic fields. What is the energy Ø Energy density of electric field, density of a magnetic field? It is found that UE = UB. Also, Uav = UE + UB = 2 UE = 2 UB = 0 E 2 = B 2/ 0.

ELECTROMAGNETIC WAVES What Intensity (I): is the intensity of The electromagnetic energy intensitycrossing of the electromagnetic per unit area per radiation unit time, from perpendicular an isotropic topoint the direction source atwaves? of a distance propagation r is, of EM wave is called intensity. s P = power of the source. r

ELECTROMAGNETIC WAVES Generation of EM Waves Ø An EM wave (radio wave) can be generated by using an AC voltage source connected to two wires. Ø The two wires act as an antenna. Ø As the voltage of the AC source oscillates, the electric potential of the two wires also oscillate. Ø Electric charges are also flowing into and off the wires as the voltage alternates.

ELECTROMAGNETIC WAVES Ø The electric field continues to oscillate in size and direction. Ø The wave propagates away from the antenna. Ø The charges are accelerated. Ø The charges undergo simple harmonic motion with a given frequency which is also the frequency of the AC voltage source and the frequency of the wave.

ELECTROMAGNETIC WAVES Ø A simple LC oscillator and energy source can produce waves of desired frequency. Now, let us see the nature of Nature of EM waves electromagnetic waves. Ø The EM waves are transverse in nature. They do not require any material medium for their propagation.

ELECTROMAGNETIC WAVES Ø Its unit is Wm-2. Ø The medium offers opposition to the propagation of wave. Such a hindrance is called Wave Impedance (Z) and its value in a medium is given by,

ELECTROMAGNETIC WAVES [ E = CB] { = r 0 and = r 0 } Where r is relative permeability and r is relative permittivity of the medium.

ELECTROMAGNETIC WAVES 1. The speed of EM waves travelling through vacuum is. . a) 3 108 m/s b) speed of light c) both (a) and (b) d) none of these MCQ S

ELECTROMAGNETIC WAVES 2. The medium offers opposition to the propagation of wave. Such a hindrance is called. . . a) wave intensity b) electromagnetic wave c) wave impedance d) none of these

ELECTROMAGNETIC WAVES 3. The energy crossing per unit area per unit time, is. . . to the direction of propagation of EM wave is called intensity. a) parallel b) perpendicular c) equal d) none of these

ELECTROMAGNETIC WAVES PROPERTIES OF EM WAVES

ELECTROMAGNETIC WAVES Properties of EM waves: Ø Momentum: EM waves also carry momentum, if a portion of EM wave of energy U propagates with speed What are the C, then linear momentum = energy (U) / speed (C). properties of electromagnetic Ø When the incident EM wave is completely absorbed by a surface, waves? it delivers energy U and momentum U/C to the surface.

ELECTROMAGNETIC WAVES Ø When a wave of energy U is totally reflected from the surface, the momentum delivered to the surface is 2 U/C. Radiation pressure(Pr): For a perfectly reflecting surface, Pr = 2 S/C; S is the Ø It is the momentum imparted per second per unit area on which the pointing vector and C is light falls. speed of light. For a perfectly absorbing surface Ø The radiation pressure is real that is why. Ptails of comet points away a = S/C. from the sun.

ELECTROMAGNETIC WAVES Ø When electromagnetic waves incident on a surface and the total energy transferred to the surface in a time t is U, then the magnitude of the momentum delivered to the surface is, complete absorption where C = velocity of light Ø When the sun shines on the hands of a person, he feels the absorption of electromagnetic waves (hands get warm).

ELECTROMAGNETIC WAVES Ø When the radiation incident on a surface is entirely reflected back along its original path, the magnitude of the momentum delivered to the surface is, where C = velocity of light Ø When radiation incident on a surface, the pressure exerted on the surface by the electromagnetic wave is equal to the radiation pressure(Pr).

ELECTROMAGNETIC WAVES (total reflection is back along the incident path)

ELECTROMAGNETIC WAVES Radiation pressure in terms of E and B, Ø Electromagnetic waves obey the principle of superposition. Ø The electric vector of an Electromagnetic field is responsible for all optical effects. For this reason, electric vector is also called a light vector.

ELECTROMAGNETIC WAVES 1. Electromagnetic waves obey the principle of. . . a) superposition b) conservation of momentum c) both (a) and (b) d) none of these MCQ S

ELECTROMAGNETIC WAVES 2. When electromagnetic waves strike a surface, a pressure is exerted on the surface is called. . . a) low pressure b) high pressure c) radiation pressure d) none of these

ELECTROMAGNETIC WAVES ELECTROMAGNETIC SPECTRUM

ELECTROMAGNETIC WAVES ELECTROMAGNETIC SPECTRUM: The orderly distribution of electromagnetic waves (according to wave length or frequency) in the form of distinct groups with different properties is called the electromagnetic spectrum.

ELECTROMAGNETIC WAVES EM Spectrum Ø There is no strict lower or upper limit for electromagnetic wave frequencies. Ø The range of frequencies assigned to the different types of waves is somewhat arbitrary. Ø Regions may overlap. Ø The names of the different regions were chosen based on how the radiation in each frequency interacts with matter and on how it is generated.

ELECTROMAGNETIC WAVES EM Spectrum Uses of EM Spectrum Radiation -rays X-rays UV-rays Visible light Infrared rays Uses Microinformation waves & Gives on nuclear structure, medical radio waves treatment etc. are used for …. ? Medical diagnosis and treatment study of crystal structure, industrial radiograph. Preserve food, sterilizing the surgical instruments, detecting the invisible writings, finger prints, etc. Radar Telecommunication. To see objects. To treat muscular strain, for taking photography in fog etc.

ELECTROMAGNETIC WAVES Radio waves Micro waves Infra-red rays Gamma Rays X-rays Visible Light Ultra violet rays

ELECTROMAGNETIC WAVES 1. The orderly distribution of electromagnetic waves according to wavelength or frequency in the form of distinct groups having widely different properties is called. . . a) intensity of radiation b) radiation pressure c) electromagnetic spectrum d) both (b) and (c) MCQ S

ELECTROMAGNETIC WAVES 2. Infrared rays are used in. . . a) finger prints b) for taking photography in fog c) treatment of muscular strain d) both (b) and (c)

ELECTROMAGNETIC WAVES Thank you…