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Chapter 5 The Nature of Light Light is a wave and white light is composed of many different wavelengths White light is just a small part of the entire electromagnetic spectrum. Continuous Emission The Sun is a source of continuous emission at T=5800K The Sun (and other Stars) radiate like Blackbodies Wiens Law Relates the wavelength of brightest emission to the temperature max T = 2.898 x 10-3 max (m) for a blackbody of temperature, T, in Kelvins. The Color of a Star is Related to it’s Temperature Saturn is a source of continuous emission by reflected sunlight Continuous Infrared emission from Io’s Volcanoes Continuous Radio Emission from a Galaxy There are different mechanisms that give rise to continuous emission but they are all related to the atomic nature of matter Electrons orbiting the Nucleus of an Atom Spectroscopy The Sun (and other Stars) radiate like Blackbodies An absorption line is like a silhouette Classic absorption spectrum for the Sun At the subatomic level, light behaves like a particle Stellar Spectra Emission & Absorption Lines Electronic Transitions in the Hydrogen Atom Energy Level Diagram Two important facts about Hydrogen • The ionization potential = 13.6 eV • The wavelength of the Ha emission line is 6563Å where 1Å = 10-10 m You can figure out everything about Hydrogen from these two facts and knowing that the energy difference between two electronic states, DE, is proportional to DE a 1/n2 Where n is the principal quantum number For Example • The Ha absorption line results from the electron jumping from • the n=2 to n=3 level. We can use this fact (#2) to calculate • the constant of proportionality, R DE = hc/= constant [ 1/ n12 – 1/n22 ] So that, 1/ = R [ 1/ n12 – 1/n22 ] Substitute n1= 2 and n2 = 3 and = 6563Å to yield R = 1.097 x 10-3 which is known as the Rydberg Constant Now you can calculate the wavelength for all other electronic transitions Since, 1/ = 1.097 x 10-3 [ 1/ n12 – 1/n22 ] But, remember, that this equation yields in Å The origin of emission and absorption lines is related to the type of background source. Some of the most beautiful objects, however, are emission (not absorption), nebulae The red glow is due to the emission from Hydrogen atoms dropping from the n=3 to n=2 level Here, the blue glow is from Oxygen atoms Intervening dust grains scatter the blue light and preferentially transmit the red light, which is why we have red sunsets. The scattered light leads to blue reflection nebulae Dust also absorbs blue light making more distant objects appear redder One of the best known Hubble pictures includes elements of all the light phenomena we have talked about