Chapter 20 Electromagnetic Waves The Waves The Wave equations Electromagnetic Spectrum Poynting Vector s
Chapter 20 Electromagnetic Waves ◼ The Waves ◼ The Wave Equations ◼ Electromagnetic Spectrum ◼ Poynting Vector S
20.1 Waves ave on a stretched string See Fig 20- Figure 20-1 Wave on a stretched string
Usey to denote the hight of the string. Then gen erally it is a function of both the time t and the coordinate 2 Moreover, as a wave. it is a function as u(t-2/u), where v is the speed of the wave. SO, tor instance, the hight of z at the time t is equal to the hight of x=0 at an earlier time t-v 0(t-x/u)=y(t-21/)-0/0)
Usey to denote the hight of the string. Then gener- ally it is a function y(t, a)of both the time t and the coordinate 2. Moreover, as a wave. it is a function as where v is the speed of the wave. So, for instance the hight of z at the time t is equal to the hight of x=0 at an earlier time t-x/v (t-x/)=y(-2/)-0/) Remarks Waves can be in air. water, vacuum or other media The quantity propagated can be either a scalar, a vector, and a tensor
An oscillating quantity a can be written as a= ao cos(wt), where ao is the amplitude, w is the angular fre- q ueno cy, f=w/ 2T is the frequency, and T=1/f is the period a plane wave a =a(t-0)travelling along the x-axis can be written as a= an coslwlt The quantity w(t-i) in the bracket is the phase The wave is also unattenuated since an is a constant