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Light is a WAVE Electromagnetic Spectrum and EM Waves EM Waves • Light is part of the ELECTROMAGNETIC SPECTRUM • EM WAVES are radiated by ALL objects at the SPEED OF LIGHT • c = 3 x 108 m/s (in vacuum) Example v = fλ #1 What is the wavelength of a radio wave traveling 8 4 3 x 10 m/s = (5 x 10 Hz) λ through outer space with a frequency of 5 x 104 Hz? λ = 600 m EM Wave Production Changing electric fields produce MAGNETIC FIELDS Changing magnetic fields produce ELECTRIC FIELDS The result is a selfpropagating wave with perpendicular ELECTRIC and MAGNETIC fields These waves DO NOT REQUIRE A MEDIUM EM Wave Speed • Depends on the Example medium #2 What is the speed of light in lucite? c • The “speed” ofn anmedium depends on its = 1.50 density and is described by its INDEX OF v REFRACTION (n) n=c/v Index of Refraction = speed light 1.50 = (3 x 108 of m/s) / vin vacuum speed of light in faster medium • The closer the index is to 1, the the medium v = 2 x 108 m/s Wein’s Law EM Waves ALL objects emit EM radiation with a frequency directly related to their temperature Outer space 3K Humans 310K Sun 5630K Example #3 • Red light has a wavelength of about 750 nm. Violet light has a wavelength of about 400 nm. Betelguese – Which color of light is emitted with a higher frequency? • Shorter wavelength = higher frequency, so violet is higher – Which star is hotter, Betelgeuse or Rigel? How can you tell? • Wein’s Law says that hotter objects emit HIGHER frequencies, so blue stars are hotter – red ones are cooler Rigel EM Waves Summary • EM waves travel at THE SPEED OF LIGHT. – That speed depends on the MEDIUM OF TRAVEL • EM waves do not require a MEDIUM. • Low frequency EM waves (below ultra violet) are NON-IONIZING forms of radiation • High frequency EM waves (ultra violet and higher) are IONIZING forms of radiation – Dangerous to life mutations, cancer, etc.