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