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.