1. Blackbody Spectrum
Remember that EMR is characterized by wavelength (frequency)
Spectrum: distribution of wavelength (or frequency) of some EMR
Blackbody: the distribution of EMR at equilibrium with matter
Equilibrium means that matter and EMR exchange the same amount of energy
Blackbody emission is a continuum: all wavelengths are present, although with different intensity

3. The Difference Between Black and White
“White” light – contains all the frequencies of the visible part of the spectrum.
White paint – diffusely scatters all frequencies of the visible part of the spectrum equally.
Black paint – absorbs all frequencies of the visible part of the spectrum equally.
“Blackbody” – emits and absorbs radiation over a specific set of frequencies.

4. Discussion Question Why does NASA paint spacecraft white? Absorption
Frequency
Infrared
Visible
Absorption Spectrum of White Paint
40% 0%
Absorption Spectrum of Black Paint
80%

5. Hotter objects emit photons with a higher average energy.
Wien’s Law
The peak of the blackbody emission spectrum is given by
Introduce temperature units, thermometer
The higher the temperature, the shorter the wavelength, i.e. the bluer

6. Thermal Radiation
Thermal radiation is basically Black-body radiation, or nearly so
Every object with a temperature greater than absolute zero emits radiation.
Hotter objects emit more total radiation per unit area.
Hotter objects emit photons with a higher average energy.
Thermal Radiation (BB) is an example of “continuum emission”.

8. Blackbody Radiation

9. The graph below shows the blackbody spectra of three different stars
The graph below shows the blackbody spectra of three different stars. Which of the stars is at the highest temperature?
1) Star A
2) Star B
3) Star C A B
Energy per Second C
Wavelength

10. Hotter objects emit more total radiation per unit surface area.
Stefan-Boltzmann Law:
Emitted power per unit area = σ T4
σ = 5.7 x 10-8 W/(m2K4)

11. 2) is bluer when hotter. 3) is both 4) is neither
You are gradually heating up a rock in an oven to an extremely high temperature. As it heats up, the rock emits nearly perfect theoretical blackbody radiation – meaning that it
1) is brightest when hottest.
2) is bluer when hotter.
3) is both
4) is neither

12. For star: L=A T4 , A is its surface area

13. Campfires
Campfires are blue on the bottom, orange in the middle, and red on top.
Which parts of the fire are the hottest? the coolest?
As atoms get hotter, they wiggle faster and collide to each other harder ---> more light they emit and more wiggle per second = frequency goes up = wavelength goes down. Thus bluer.
Thus, as temperature goes up light gets stronger and gets blue.

14. How to Make Light (Part 2)
Structure of atoms
Energy levels and transitions
Emission and absorption lines
Light scattering
Please pick up a diffraction grating on your way into class.

15. Atoms Atoms are made of electrons, protons, and neutrons.
A Planetary Model of the Atom
Atoms are made of electrons, protons, and neutrons.
The bounding force: the attractive Coulomb
(electrical) force between the positively charged nucleus and the negatively charged electrons.
A story about atoms
To understand the microscopic processes involved and to study another emission mechanism
As an introduction to elements

16. Energy Levels
Electrons can be in different orbits of certain energies, called energy levels.
Different atoms have different energy levels, set by quantum physics.
Quantum means discrete!
Ladder
Gasoline vs apple

17. Excitation of Atoms
To change its energy levels, an electron must either absorb or emit a photon that has the same amount of energy as the difference between the energy levels
E = h = h c/l
Larger energy difference means higher frequency.
Different jumps in energy levels means different frequencies of light absorbed.

18. Spectral Line Emission
If a photon of exactly the right energy is absorbed by an electron in an atom, the electron will gain the energy of the photon and jump to an outer, higher energy orbit.
A photon of the same energy is emitted when the electron falls back down to its original orbit.

20. Spectral Line Emission
Collisions (like in a hot gas) can also provide electrons with enough energy to change energy levels.
A photon of the same energy is emitted when the electron falls back down to its original orbit.

21. Energy Levels of a Hydrogen Atom
Different allowed
“orbits” or energy levels in a hydrogen atom.
Emission line spectrum
Absorption line spectrum

23. Spectral Lines of Some Elements
 Argon 
                                                                                
 Helium
                                                                                 
 Mercury
 Sodium
 Neon
Spectral lines are like a cosmic barcode system for elements.

24. Atoms of different elements have unique spectral lines because each element
has atoms of a unique color
has a unique set of neutrons
has a unique set of electron orbits
has unique photons

25. Blackbody radiation

26. The Solar Spectrum
There are similar absorption lines in the other regions of the electromagnetic spectrum. Each line exactly corresponds to chemical elements in the stars.

28. Emission nebula
Reflection nebula

29. An Object’s Spectrum
Encoded in an object’s spectrum is information about the emitter/absorber. This is how we learn what the Universe
is made of!
This technique is so reliable that scientists can tell what elements they are looking at just by
reading the lines. Spectroscopy is the science of using spectral lines to figure out what something is made of. That's how we know the composition of distant stars, for instance.