Photon_Atomic_Quantum_PhotoElectriEffect.ppt

Atomic Theory, Quantum Theory, Photoelectric effect, High school Physics, blackbody spectrum, photon theory

1. Chapter 27-Atomic/Quantum
Physics

2. The Sun
http://soho.nascom.nasa.gov/
Why do we see the sun as yellow
instead of green or blue or pink?

3. Blackbody Spectrum

4. Photon Theory of Light
Light is transmitted as tiny particles
called photons
The amount of energy in a photon
depends on its frequencypc
hc
hfE 

h= Planck’s Constant= 6.626 x 10
-34
Js
f= frequency of light
c= speed of light
λ= wavelength
p= momentum

5. The Photoelectric Effect
When light shines on a metal surface,
electrons are emitted from the surface

6. 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

7. Photocells
KE
maxof 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,
V
o, called the stopping
voltage. No current will flow if
the voltage is less than the
stopping voltage
Ke
max= e V
o

8. Photoelectric Effect
Einstein’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 electronsoo WhfhfhfKE 
max

9. Photoelectric Effect
The work function, W
o, is the minimum
amount of energy necessary to get an
electron off the surface of the metal
f
ois the “cutoff” frequency. If the light
beam’s frequency is below that, then
no electrons will be emittedoohfWE 

10. Compton Effect
A.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

11. de Broglie Wavelength
Light sometimes behaves like a wave
and sometimes like a particle
Louis de Broglie came up with the
idea that particles might also have
wave propertiesmv
h
p
h

De Broglie wavelength of a particle

12. de Broglie Wavelength
The wavelength of large objects is
very small
For a 0.20 kg ball travelling at 15 m/s  
mx
smkg
Jsx
34
34
102.2
/1520.0
106.6




13. de Broglie Wavelength
Determine the wavelength of an
electron that has been accelerated
through a potential difference of 100 V2
2
1
mvqV s
m
x
m
qV
v
6
109.5
2
 mx
s
m
xkgx
Jsx
mv
h
10
631
34
102.1
)109.5)(101.9(
1026.6





14. Davisson-GermerExperiment
The 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

15. Bohr Model of the Atom
Electrons 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

16. Bohr Model of the atom
If an electron jumps from a higher
state to a lower state, it emits a single
photon of light statelower ofEnergy -stateupper ofEnergy EnergyPhoton 
luEEhf

17. Energy Level Diagram (p.
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)

18. Energy Level Diagram
How 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 

19. Energy Level Diagrams
What are the
possible transitions
for an electron in
excited state n=3?
◦31
◦32
◦21

20. Emission Spectra
A material’s emission spectrum show the
wavelengths of the photons emitted when
electrons jump to lower energy states

21. Absorption Spectra
The absorption spectrum of a material
shows that gases can absorb light at
the same frequencies at which they
emit

22. Absorption/Emission Spectra