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CHAPTER 3: Light and Telescopes Electromagnetic Radiation Only a very small range of wavelengths, 400nm to 700nm, are visible to humans. Electromagnetic radiation waves, are fluctuations of electric and magnetic fields, which move through empty space at the speed of light Wavelengths are very small so astronomers use nanometers 10 -9 m or angstroms 10 -10 m Wave Properties of Light Frequency f - number of waves that arrive in a given time Wavelength λ - distance between crests Velocity v - distance covered in one second by a crest in a certain direction fxλ=ν velocity of light in a vacuum c = 300,000km/s fxλ=c The Doppler Effect Sources moving toward the observer squeeze light waves in front of them, causing them to be shorter. We call this a blueshift. Sources moving away from the observer stretch the light waves behind them, causing them to be longer. We call this a redshift. The transparency of a material depends on the wavelength of light. Earth’s atmosphere is relatively transparent to visible light and radio waves, which are referred to as “windows” through which we can view space from a ground-based telescope. Particle Properties of Light Radiating matter emits light in discrete chunks of energy called QUANTA (photon) When atoms are excited they can jump up energy levels- when they move back down they give off photons with energy equal to the amount it needs to loose Energy(photon) = hc/ λ h= (planck’s constant = 6.63 x1023 J s CRAB NEBULA Optical Ultra Violet X-Ray Radio Multiwavelength Universe Radiation Type Temp. Source Gamma-rays more than 108 (K) accretion disks around black holes X-rays 106-108 K gas in clusters of galaxies, supernova remnants; stellar corona Ultraviolet 104-106 K supernova remnants; very hot stars Visible 103-104 K planets, stars, some satellites Infrared 10-103 K cool clouds of dust and gas; planets Radio less than 10 K radio emission produced by electrons moving in magnetic fields