how is light produced?

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.