how is light produced?
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
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 gaseous
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=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.
http://www.oswego.edu/~kanbur/a100/images/planck.jpg
(Assuming stars are
same size.)
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).
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 gaseous
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/lectures/lecture19/pics/emission_spectra.gif
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
• 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.