# How is Light Produced? It’s all tied to energy Energy of the material’s temperature Energy levels within atoms.

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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

http://www.oswego.edu/~kanbur/a100/images/planck.jpg (Assuming stars are same size.)

Planck’s Law Completely describes the light (blackbody radiation) coming from an object.

Stefan-Boltzmann Law L=σT 4 ×star’s surface area The total brightness of an object (at all colors added together) depends on the Temperature to the 4 th power (and size of the object). Temperature makes objects glow. The hotter it is, the more it glows.

http://www.oswego.edu/~kanbur/a100/images/planck.jpg (Assuming stars are same size.)

Wein’s Law λ max =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).

http://hypertextbook.com/physics/modern/planck/ Hottest stars look blue Our Sun looks yellow Cool stars look red (Assuming same size stars.)

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

Johann Jakob Balmer Swiss 1825-1898 http://upload.wikimedia.org/wikipedia/commons/5/51/Balmer.jpeg

http://homepage.mac.com/dtrapp/chemGraphics.f/HydrogenSpectra.jpg

Balmer series Hydrogen puts out light in very specific colors Wavelengths of colors related to 1/2 2 -1/n 2, where n=3, 4, 5… Balmer did not yet understand why.

Johannes Rydberg Swedish 1854-1919 http://zahav.elementy.ru/images/eltbio/rydberg_johannes_robert_160.jpg

http://upload.wikimedia.org/wikipedia/commons/thumb/5/55/Bohr- atom-PAR.svg/310px-Bohr-atom-PAR.svg.png When an electron goes down an orbital

http://upload.wikimedia.org/wikipedia/commons/thumb/5/55/Bohr- atom-PAR.svg/310px-Bohr-atom-PAR.svg.png A photon comes out of a specific color

Bohr Atom and Energy Levels Each level of electron shell corresponds to an amount of energy. Electrons can jump from one shell to another by emitting or absorbing a photon. The frequency (color) of the photon indicates the energy difference between the levels (E=h ν ).

http://www.daviddarling.info/images/hydrogen_spectrum.gif

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.

http://upload.wikimedia.org/wikipedia/commons/thumb/5/55/Bohr- atom-PAR.svg/310px-Bohr-atom-PAR.svg.png When an electron goes down an orbital

http://upload.wikimedia.org/wikipedia/commons/thumb/5/55/Bohr- atom-PAR.svg/310px-Bohr-atom-PAR.svg.png A photon comes out of a specific color

Absorption Spectrum Light of all colors comes in. When the color is just right, it makes the electron pop up to a higher level.

http://upload.wikimedia.org/wikipedia/commons/thumb/5/55/Bohr- atom-PAR.svg/310px-Bohr-atom-PAR.svg.png Light of all colors comes in

http://upload.wikimedia.org/wikipedia/commons/thumb/5/55/Bohr- atom-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://www.physics.umd.edu/courses/Phys401/bedaque06/discrete_spectra.jpg bright emission lines become dark absorption lines

Conclusion Colors of light (how many colors and how bright) call tell us the temperature, density, composition, and even shape of an object.

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