EM WAVES
 

I. Electromagnetic waves (EM waves) consist of oscillating E- and B-fields that can usually be described using sines and cosines.

  A. They include radio, TV, infrared, visible, ultraviolet, X-rays, and gamma rays.

  B.The wavelengths for visible light range from about 400 nm (violet) to 700 nm (red) (1 nm = 1 X 10-9 m)

  C. EM waves are radiated whenever charges accelerate or oscillate.

II. EM wave properties

  A. The oscillating E and B fields are perpendicular to each other and to the direction of wave travel.

  B. The cross product E x B is proportional to the Poynting vector (S) which "points" in the direction of wave travel and gives   the direction of energy flow.

  C. The EM waves all have the same speed in vacuum: c = 3.0 x 108 m/s

  D. The amplitudes of the E- and B-waves have the ratio: E/B = c

  E. EM waves have linear momentum and can exert a radiation pressure on an object

   1. For total absorption: pr = I/c where I is the intensity of the EM wave

   2. For total reflection back along the incoming path: pr = 2I/c where I is the intensity of the EM wave

   3. Note that intensity decreases with distance from the source

    (a) For a point source: I = Ps/4(pi)r2