chapter 27- atomic/quantum physics. the sun why do we see the sun as yellow instead of green or...
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Chapter 27- Chapter 27- Atomic/Quantum PhysicsAtomic/Quantum Physics
The SunThe Sunhttp://soho.nascom.nasa.gov/
Why do we see the sun as yellow instead of green or blue or pink?
Blackbody SpectrumBlackbody Spectrum
Photon Theory of LightPhoton Theory of LightLight is transmitted as tiny
particles called photons
The amount of energy in a photon depends on its frequency
pchc
hfE
h= Planck’s Constant= 6.626 x 10-34 Jsf= frequency of lightc= speed of lightλ= wavelengthp= momentum
The Photoelectric EffectThe Photoelectric EffectWhen light shines on a metal
surface, electrons are emitted from the surface
Photocells (p.829)Photocells (p.829)When the
photocell is in the dark, the ammeter reads 0 (no current)
When light with a high enough frequency shines on the current flows in the circuit
PhotocellsPhotocellsKEmax of the emitted electrons
can be found by reversing the voltage and making the C electrode negative
The electrons are repelled by C, but the fastest electrons will still make it across
There is a minimum voltage, Vo, called the stopping voltage. No current will flow if the voltage is less than the stopping voltage
Kemax = e Vo
Photoelectric EffectPhotoelectric EffectEinstein’s Theory Predicts:
◦ Increasing the intensity of the light does not make the electrons go faster. This is because although more photons are striking
the surface, they have same energy
◦ Increasing the frequency of the light beam increases the energy of the photons which changes the maximum KE of the ejected electrons
oo WhfhfhfKE max
Photoelectric EffectPhotoelectric EffectThe work function, Wo, is the
minimum amount of energy necessary to get an electron off the surface of the metal
fo is the “cutoff” frequency. If the light beam’s frequency is below that, then no electrons will be emitted
oo hfWE
Compton EffectCompton EffectA.H. Compton scattered
xrays from various materials◦ Found out that the
scattered light had a lower frequency than incident light
◦ Since frequency decreases, wavelength increases
Used conservation of momentum to determine that the photon transfers some of its energy to the electron
de Broglie Wavelengthde Broglie WavelengthLight sometimes behaves like a
wave and sometimes like a particle
Louis de Broglie came up with the idea that particles might also have wave properties
mv
h
p
h
De Broglie wavelength of a particle
de Broglie Wavelengthde Broglie WavelengthThe wavelength of large objects
is very small
For a 0.20 kg ball travelling at 15 m/s
mxsmkg
Jsx 3434
102.2/1520.0
106.6
de Broglie Wavelengthde Broglie WavelengthDetermine the wavelength of an
electron that has been accelerated through a potential difference of 100 V
2
2
1mvqV
s
mx
m
qVv 6109.5
2
mx
sm
xkgx
Jsx
mv
h 10
631
34
102.1)109.5)(101.9(
1026.6
Davisson-Germer Davisson-Germer ExperimentExperimentThe spacing of atoms in a crystals
is on the order of 10-10 m, so one could be used as a diffraction grating
In 1927, Davisson and Germer scattered electrons from the surface of a metal crystal. The wavelength they got matched the predicted de Broglie wavelength
Bohr Model of the AtomBohr Model of the AtomElectrons orbit the nucleus in
circular orbits called stationary states
When an electron jumps from one state to another, light is either absorbed or emitted
The energy required to go between states is a fixed amount
Bohr Model of the atomBohr Model of the atomIf an electron jumps from a
higher state to a lower state, it emits a single photon of light
statelower ofEnergy -stateupper ofEnergy EnergyPhoton lu EEhf
Energy Level Diagram (p. Energy Level Diagram (p. 847)847)
n= 1is ground state, n=2,3,4.. Are excited states
To completely free an electron in the ground state, you’d need to put in 13.6 eV of energy (ionization energy of Hydrogen)
Energy Level DiagramEnergy Level DiagramHow much
energy to go from ground to n=2?
How much energy to go from n=2 to n=4?
eVeVeV 2.104.36.13
eVeVeV 55.285.04.3
Energy Level DiagramsEnergy Level DiagramsWhat are the
possible transitions for an electron in excited state n=3?◦31◦32◦21
Emission SpectraEmission SpectraA material’s emission spectrum
show the wavelengths of the photons emitted when electrons jump to lower energy states
Absorption SpectraAbsorption SpectraThe absorption spectrum of a
material shows that gases can absorb light at the same frequencies at which they emit
Absorption/Emission Absorption/Emission SpectraSpectra