Characteristics of electromagnetic waves are as follows :
(1) In electromagnetic waves electric field and magnetic field are perpendicular to each other as well as perpendicular to direction of propagation.
During charging process of capacitor electric field inside plate is perpendicular to plate and magnetic field produced due to displacement current is circular and parallel to close loop and in direction of plate hence `vec(E )` and `vec(B)` are perpendicular to each other.
(2) Let plane electromagnetic wave propagates in z - direction. Here, electric field `E_(x )` is in x - direction and electromagnetic wave propagate in z - direction and varies according to sine function. Electric and magnetic field `E_(x)` and `B_(y)` are perpendicular to each other and perpendicular to direction of propagation (z - axis). Hence, it can be represented mathematically,
`E_(x)=E_(0)sin (kz-omega t) " "` ....(1)
`B_(y)=B_(0)sin(kz-omega t)" "` ....(2)
`therefore vec(E )=E xi+0J+0hat(k)=E_(0)sin(kz-omega t)hat(i)`
and `vec(B)=0+Byj+0hat(k)=B_(0)sin(kx-omega t)hat(j)`
where k = wave vector `=(2pi)/(lambda) " "` .....(3)
It shows direction of propagation of wave.
`omega =` angular frequency
`(omega)/(k)=` wave speed
`omega = ck " "` ....(4)
where `c=(1)/(sqrt(mu_(0)in_(0)))`
`omega = ck` is standard equation for waves
`pi v=c xx(2pi)/(lambda)[because omega = 2pi v, k=(2pi)/(lambda)]`
can be written as,
`therefore v lambda = c " "` ....(5)
(3) In waves electric and magnetic field are related by equation,
`(E_(0))/(B_(0))=c rArr B_(0)=(E_(0))/(c )" "` .....(6)
(4) In free space oscillation of electric field and magnetic field medium is not required where as other waves require medium for propagation.
Example sound waves in air, sound waves in solid medium, transverse waves produced on water.
In nineteenth century scientists were of the belief that medium is required for propagation of waves. They tought that universal medium like ether should be present everywhere for propagation of oscillation of electric and magnetic field. In 1887 Michelson and Moley by doing experiment discarded concept of ether as medium.
Now, it is accepted that no physical medium is required for propagation of electromagnetic waves. Oscillation of electric and magnetic field are continuous and self sustained in vacuum.
(5) Electromagnetic waves are transverse and non - mechanical waves.
(6) Speed of electromagnetic waves in free space or vacuum is important constant and this constant is independent of wave length. Its value is `3xx10^(8)m//s`.
By using this speed stand unit of length (meter) is defined.
Distance travelled by light in vacuum in `(1)/(c )=(1)/(2.99792458xx10^(8))` second is called 1 meter.
Thus unit of length can be expressed in terms of unit of c and time.
(7) Scientist showed that electromagnetic waves experiences diffraction, refraction and polarization.
Hertz produced stationary electromagnetic waves by using distance between two nodes he measured wavelength and knowing frequency from equation `v = v lambda` measured speed which was to be equal to speed of light.
(8) Electromagnetic waves are polarized which can be seen from attention of waves from small AM radio broadcasting station.
(9) Electromagnetic waves transmit energy and momentum.
In free space in region of electric field E energy density of electric field is `(rho_(E )=(1)/(2)E_(0)E^(2))` and energy density of magnetic field is `(rho_(B)=(1)/(2)(B^(2))/(mu_(0)))`.
Energy density of electromagnetic waves (energy per unit volume) is `rho`, then
`rho = rho_(E )+rho_(B)`
`therefore rho=(1)/(2)in_(0)E^(2)+(1)/(2)(B^(2))/(mu_(0))` which is for steady electric and magnetic field.
In above equation,
`rho=(1)/(2)in_(0)E^(2)+(1)/(2)(B^(2))/(mu_(0)), B=(E )/(c )` and `c=(1)/(sqrt(mu_(0)in_(0)))`
`rho=(1)/(2)in_(0)E^(2)+(1)/(2mu_(0))xx(E^(2))/(1//mu_(0)in_(0))`
`therefore rho =(1)/(2)in_(0)E^(2)+(1)/(2)in_(0)E^(2)`
`therefore rho = in_(0)E^(2)`
Which is equation of electromagnetic energy density.
(10) When electromagnetic waves collide on a surface it exert pressure on the surface which is called radiation pressure.
Energy incident on any surface in time t be U and this energy is completely absorbed then momentum imparted,
`p=(U)/(c )`
momentum imparted on our hand by sunlight is very less hence we do not experience pressure.
In 1903 American scientist Nicols and Hull measured radiation pressure of sunlight which was order of `7xx10^(-6)N//m^(2)`. This means that force acting on `10 cm^(2)` area due to radiation pressure is `7xx10^(-9)N`.
(11) Electromagnetic waves carry energy hence from transmitting station it can carry radio and TV signals.
(12) Electromagnetic waves carry energy from the sun to earth hence life sustain on the earth.
(13) In region far from source oscillation of electric and magnetic field are in same phase.
(14) Direction of `vec(E )xxvec(B)` is in direction of propagation of electromagnetic waves.
