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ATTENTION:
EXAM next FRIDAY (one week)!! * Exam covers the reading Chapters 1-6* Sample questions on the web.
HW – due Wednesday midnight
Atom-photon interactions
hc
fhE Photons:
Atom Light
Energy exchangedquantized
Most commonly by 1 photon
“photons”
Atom Light
Absorption
Atom Light
Emission
Absorption
Ground state
Atoms in ground state initially
Absorption
Ground state
hf /19.10
Have to have a photon present of the right frequency (n =1 → 2)
One photon absorbed
Absorption
Ground state
hf /74.12One photon absorbed
Have to have a photon present of the right frequency (n =1 → 4)
Another possibility …
White light H atoms
slit
Absorption spectrum
Dark lines appear in spectrum at frequencies (energies) where photons
can be absorbed by the atom
Emission
Ground state
Atom must be in excited state
Emission
Ground state
hf /55.2One photon emitted
Have to have an atom present in the n = 4 excited state (n = 4 → 2)
Emission
Ground state
hf /74.12One photon emitted
Have to have an atom present in the n = 4 excited state (n = 4 → 1)
Another possibility …
H atoms“Hot”
slit
emission spectrumHave to Jazz the atoms up somehow into their excited states
Bright spectral lines appear only at frequencies (energies) that the atoms can
emit
(Kirchhoff’s Law’s 2 and 3)
A diffuse gas emits or absorbs light only at discrete frequencies specific to the substance of the gas
Goofy-glasses demo: emission and absorption spectra
Matter in generalMore degrees of freedom more energy levels
Simple atomic Complicated molecule
lines ‘bands’
“white light”
A white object doesn’tabsorb anything
Reflective colors
WALL
“white light”
A red object ‘looks’Red because it absorbsThe blue and greens
Reflective colors
WALL
Why green leaves turn red
Chlorophyll dies/reabsorbed in autumn leaving longer-lived Carotenoids
If an object can emit light at a certain wavelength,then it can also absorb light at that same wavelength!
Thermal radiation from continuous sources
Recall …
Cavity radiation
T
Cavity radiation
T
Cavity radiation
T
Cavity radiation
T
The ‘color’ and brightness of the light you see in the cavity depends on the relative intensities of the different wavelengths of light present.
These intensities depend on the temperature of the walls!
For cavity radiation, we can work out the distribution of intensities of all wavelengths, given the temperature T
Colored objects ….
yellow body red body
Colored objects ….
yellow body red body
Absorbs: Red, Blue, GreenReflects: yellow
Absorbs: Blue & GreenReflects: Red
An ideal black body
Black body
Absorbs light at ALL wavelengths (not just visible!)
So, it must be able to emit light at any wavelength too!
It turns out that …..
T
It turns out that …..
T
Intensity of light of a given ,l emitted from a unit area of the blackbody
It turns out that …..
T
Intensity of light of a given ,l emitted from a unit area of the blackbody
IDEAL Thermal-radiation spectra:
Most large objects are very nearly likeideal black bodies, with an associated
thermal emission spectrum.
Cool object(most intensity in IR)
The higher an object’s temperature ….
The more intensely it emits electromagnetic radiation, and …
The shorter the wavelength of the light is at which it’s emitting most strongly!
Wien’s displacement law:
T
0029.0max
in °Kin m
Stefan-Boltzmann law
4TP
in °K
Total power radiated(at all wavelengths) per unit area of the emitting body.
in Watts/m2
5.67×10-8 W/(m2 °K4)
Doppler Effect
the water-wave analogy
drooling duck
drooling duck
drooling duck
drooling duck
drooling duck
drooling duck
drooling duck
drooling duck
lrest
drooling duck
drooling duck
drooling duck
drooling duck
drooling duck
drooling duck
drooling duck
lforward
drooling duck
lforward
lrest
source
l in front
l in back
Doppler effect with light: similar behaviorbut for very different reasons!
source movingaway
source movingtoward
RED BLUE
REDBLUE
Doppler shift measures Radial velocity
Radial velocity
Radial velocity
Transverse velocity
Radial velocity
Radial velocity
Transverse velocity
0
c
VR
