How is Light Produced? • It’s all tied to energy • Energy of the material’s temperature • Energy levels within atoms Motivation • If we understand how light is produced, then when we see light we’ll know the conditions under which it was created. • Examples: – That scope thing Spock uses – Atmospheric composition of a newly discovered planet – Can a given star support life? Kirchoff’s Laws - 3 types of spectra • Continuous or Continuum – Ex: Blackbody radiation • Emission • Absorption Hot solid thing Hot gaseou s thing Transparent thing blocking other hot thing Hot dense thing Continuous Spectrum • Continuous, continuum • All colors • Examples: stars are nearly blackbody, incandescent light bulbs, electric burners, people, etc. Causes of Continuum • Blackbody – Thermal – Hot dense material • Bremsstrahlung / Free-free – An electron passes by a proton / nucleus • Recombination / Free-bound – Electron captured by a proton / nucleus • Compton Scattering – Existing photon has its wavelength changed by a collision (Assuming stars are same size.) http://www.oswego.edu/~kanbur/a100/images/planck.jpg Planck’s Law • Completely describes the light (blackbody radiation) coming from an object. Stefan-Boltzmann Law • L=σT4×star’s surface area • The total brightness of an object (at all colors added together) depends on the Temperature to the 4th power (and size of the object). • Temperature makes objects glow. The hotter it is, the more it glows. (Assuming stars are same size.) http://www.oswego.edu/~kanbur/a100/images/planck.jpg Wein’s Law • λmax=2,900,000/T (in nm) • What color an object is brightest at depends on the Temperature of the object. • Hotter objects are brightest in blue/purple (and ultraviolet). • Cooler objects are brightest in red (and infrared). Hottest stars look blue Our Sun looks yellow (Assuming same size stars.) Cool stars look red http://hypertextbook.com/physics/modern/planck/ How can you tell which object is hotter/larger 1. Color of the peak tells us the object’s temperature. 2. If two objects have the same color, the brighter one is physically larger. 3. If two objects of the same size, the hotter one will be brighter at all colors. Hot gaseou s thing Emission Spectrum • Hot thin gas • Only a few select colors • Examples: some fluorescent lights, neon lights, natural gas flames, warm gas clouds in space http://astronomy.nmsu.edu/nicole/teaching/ASTR110/lec tures/lecture19/pics/emission_spectra.gif When an electron goes down an orbital http://upload.wikimedia.org/wikipedia/commons/thumb/5/55/Bohratom-PAR.svg/310px-Bohr-atom-PAR.svg.png A photon comes out of a specific color http://upload.wikimedia.org/wikipedia/commons/thumb/5/55/Bohratom-PAR.svg/310px-Bohr-atom-PAR.svg.png Absorption Spectrum • Hot dense object blocked by cool thin gas • Continuum minus emission • All colors except a select few • Examples: nearby gas cloud blocks a farther star, nearer galaxy blocks a far quasar, sunglasses block sunlight, Earth’s atmosphere blocks sunlight Transparent thing blocking other hot thing http://www.solarobserving.com/pics/hydrogen-spectra.jpg Absorption and emission spectra are opposite in appearance and cause. Emission spectrum Electrons go down levels on their own and put out light as a result. When an electron goes down an orbital http://upload.wikimedia.org/wikipedia/commons/thumb/5/55/Bohratom-PAR.svg/310px-Bohr-atom-PAR.svg.png A photon comes out of a specific color http://upload.wikimedia.org/wikipedia/commons/thumb/5/55/Bohratom-PAR.svg/310px-Bohr-atom-PAR.svg.png Absorption Spectrum Light of all colors comes in. When the color is just right, it makes the electron pop up to a higher level. Light of all colors comes in http://upload.wikimedia.org/wikipedia/commons/thumb/5/55/Bohratom-PAR.svg/310px-Bohr-atom-PAR.svg.png Only the right color of light is used up to make the electron jump up orbitals http://upload.wikimedia.org/wikipedia/commons/thumb/5/55/Bohratom-PAR.svg/310px-Bohr-atom-PAR.svg.png bright emission lines become dark absorption lines http://www.physics.umd.edu/courses/Phys401/bedaque06/discrete_spectra.jpg Conclusion • Colors of light (how many colors and how bright) call tell us the temperature, density, composition, and even shape of an object.