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Light is a WAVE 5.2.1 EM Spectrum & EM Waves EM Waves • Charged objects generate electric fields. – When they move, the field changes. • Changing electric fields produce MAGNETIC FIELDS. Require a medium • Changing magnetic fields produce ELECTRIC FIELDS. • The E-fields and B-fields are perpendicular to each other. • Waving a charge back and forth generates an EM WAVE. Do not require a medium The Speed of Light (well, EM Waves really) • EM Waves travel at 3.0 x 108 m/s (in a vacuum). • They are slower in denser mediums. • Speed of a medium depends on INDEX OF REFRACTION. • The closer the index is to 1.0 the faster the medium. Index of Refraction Equation c n v Speed of Light in vacuum Speed of Light in medium Example What is the speed of light in a medium with an index of refraction of 2.0? n=c/v v=c/n v = (3 x 108 m/s) / (2.0) v= 1.5 x 108 m/s EM Wave Types An EM Wave’s type depends on its frequency/wavelength Radio Microwave Infrared Visible Ultraviolet X-ray Gamma Ray 103 10-2 10-5 10-7 10-8 10-10 10-12 Wavelength (m) About the size of… Building Human Honey Bee Pinpoint Protozoan Molecules Atoms Atomic Nuclei Freq (Hz) 104 Temperature of bodies emitting the wavelength 108 1012 1K 1015 100 K 10000 K 1016 1018 10 million K All objects emit EM radiation 1020 Practice 1. The constellation of Orion is a highly recognizable arrangement of stars. There are two very distinct stars in this constellation, Betelgeuse which is a red star and Rigel which is a bluish star. The color red has a wavelength of about 750 nm and the color blue has a wavelength of about 400 nm. a. Which color has the higher frequency? BLUE b. Which star is hotter, Betelgeuse or Rigel? RIGEL Practice 2. An EM wave is traveling through outer space with a frequency of 8.0 x 1013 hertz. a. What is the wavelength of this wave? v = fλ λ= v / f λ = (3.0 x 108 m/s) / (8.0 x 1013 Hz) λ = (3.8 x 10-6 m) b. What type of wave is this? Infrared Practice 3. Listed below are the properties of several EM Waves. State whether the property given is a wavelength or frequency, then state which type of EM Wave it is. a. 500000 hertz frequency _______________ radio ______________ b. 5.0 x 10-9 meter wavelength _______________ X ray / UV ______________ 3.0 x 1013 hertz frequency _______________ infrared (IR) ______________ d. 4.5 x 1014 hertz frequency _______________ visible light (red) ______________ e. 200 meters wavelength _______________ radio ______________ c. End of 5.2.1