Transcript NASC 1110
Lecture 11 Electromagnetic Waves Chapter 21.8 21.13 Outline • Discovery and Studies of Electromagnetic Waves • Properties of Electromagnetic Waves • The Spectrum of Electromagnetic Waves Discovery of EM Waves A changing magnetic field generates an electric current electromagnetic induction. J.C.Maxwell proposed that a changing electric field has an associated magnetic field. Such a combined effect results in existence of electromagnetic waves, which can travel indefinitely in empty space (vacuum). Electromagnetic waves travel with the speed of light. Properties of EM Waves The electric and magnetic fields of a wave are perpendicular to each other and to the travel direction. EM waves are transverse waves. EM waves travel with the speed of light (c). EM waves carry both energy and momentum. For an EM wave: 1 c = = Average power per unit area (0 є0) Emax Bmax Emax2 c Bmax2 = = 8 2.998 10 m/s 2 0 2 0 c 2 0 c = E/B Problem with EM waves Problem: An EM wave in vacuum has an electric field amplitude of 300 V/m. Find the amplitude of the corresponding magnetic field. c =E/B B = E/c B = 300 V/m / 3 108 m/s = 106 T = 1 T Properties of EM Waves All EM waves travel through space with a speed c their wavelength and frequency f are related: Demonstration f =c The wavelength is the distance between adjacent peaks of the electric or magnetic field 1 nm (nanometer) = 10–9 m 1μm (micron) = 10–6 m The frequency is the number of peaks that pass by any point each second, measured in cycles per second or Hertz (Hz). light demo The Spectrum of EM Waves The total variety of EM waves is called the electromagnetic spectrum. Different portions of the spectrum are called: Gamma rays the shortest wavelength light X rays - wavelengths shorter than ultraviolet The ultraviolet light - beyond the blue end of rainbow The visible light - what we see with our eyes The infrared light - beyond of the red end of rainbow Radio waves - light with the longest wavelengths Electromagnetic spectrum Electromagnetic spectrum E=h E = energy h = Planck constant = frequency Summary • Electromagnetic waves are fluctuations generated by moving charges that produce fluctuating electric and magnetic fields. • Visible light is only a small portion of the electromagnetic spectrum.