Title 08.Electromagnetic-Waves-f.pdf ✅

Physics Smart Booklet 1 8.ELECTROMAGNETIC WAVES Physics Smart Booklet Theory + NCERT MCQs + Topic Wise Practice MCQs + NEET PYQs
Physics Smart Booklet 2
Physics Smart Booklet 3 Electromagnetic Waves Electric and magnetic phenomena are not totally independent, but are related to each other. By the middle of the nineteenth century, several empirical laws of electricity and magnetism had been established. However, there was no convincing theoretical explanation for these phenomena. A comprehensive theory which could explain all the electric and magnetic phenomena was developed by James Clerk Maxwell around 1872. This theory is referred to as the ‘electromagnetic theory’. Maxwell unified not only the apparently distinct theories on electricity and magnetism but also showed that light is electromagnetic in nature. Maxwell’s equations 1. 0 q E dS  =   : Gauss’ law in electrostatics 2. B dS 0  =  L Gauss’s law in magnetism. 3. B dQ E dl dt  = −  : Faraday’s law of electromagnetic induction 4. E dl (I I ) 0 c d  =  +  : Ampere-Maxwell law. Displacement current The term Id in the fourth equation is called displacement current. E d 0 d I dt  =  Displacement current through a charging capacitor, d dV I C dt = Electromagnetic waves Electromagnetic waves consists of oscillating electric and magnetic fields. These fields oscillate in perpendicular directions, perpendicular to the direction of propagation. In a plane electromagnetic wave, travelling along z direction the oscillations of the electric and magnetic fields are represented as E E sin(K t) x 0 z = −  B B sin(K t) y 0 z = −  Here 2 k ; 2 f ; c K   =  =  =  , the speed of propagation of the waves. It can be shown that 0 0 1 c =   . Also 0 0 E c B = (a) The intensity of electromagnetic wave is given by 2 2 0 0 0 0 1 1 I E c B c 2 2 =  =  (b) The average energy density of electromagnetic wave is given by 2 2 av 0 0 0 0 1 1 U E B 2 2 =  =  (c) The momentum transferred by an electromagnetic wave on t an absorbing surface is given by I p c = . Properties of electromagnetic wave Electromagnetic wave Z X
Physics Smart Booklet 4 1. Electromagnetic waves propagate in the form of time varying electric and magnetic fields such that the two fields are perpendicular to each other and also to the direction of propagation of the waves (Figure). In other words, electromagnetic waves are transverse in nature. 2. Electromagnetic waves are produced by accelerated charges 3. Electromagnetic waves travel in free space (or vacuum) with a speed c given by c = 8 1 0 0 1 3 10 ms− =    ... (1) 0 and 0 being the permeability and permittivity of free space which have values 4 × 10–7 H m–1 and 8.85  10–12 F m–1 . The electromagnetic waves travel in a material medium of absolute permittivity  and absolute permeability  with a speed v given by 1 v =  ... (2) 4. Electromagnetic waves do not need any material medium for their propagation. 5. The ratio of the amplitudes of electric and magnetic fields is always a constant and it is equal to the velocity of electromagnetic waves. Mathematically, 0 0 E c B = ... (3) 6. The energy of electromagnetic waves is equally divided between the electric and magnetic field vectors. 7. The direction of flow of electromagnetic wave is given by the direction of a vector, called Poynting’s vector (S) . The magnitude of S gives the amount of energy flowing normally across unit area 0 E B S  =  Electromagnetic spectrum The orderly distribution of electromagnetic waves (according to wavelength or frequency) in the form of distinct groups, having widely different properties, is called electromagnetic spectrum. The spectrum of electromagnetic radiation has a wide range of wavelengths from radio-waves at one end to -rays at the other end. The spectrum from about 400 nm to 700 nm which is sensitive to human eye is called the visible spectrum. The spectrum is divided into various regions depending on the effects produced by them. A sequence of regions in the decreasing order of wavelengths is; radio waves, microwaves, infra-red rays, visible light, ultraviolet rays, X-rays and -rays. Table below gives details about the frequency range, mode of production and use of different types of electromagnetic waves in the order of decreasing wavelength.