CH302 Vanden Bout/LaBrake Fall 2012
Vanden Bout/LaBrake
CH301
ELECTROMAGNETIC RADIATION
UNIT 2 Day 1
CH302 Vanden Bout/LaBrake Spring 2012
Important Information
EXAM GRADES WILL BE POSTED By SATURDAY MORNING
LM12 due Tue 9AMHW4 due Tue 9AM
UNIT2DAY1-VDBWednesday, September 19, 201210:18 PM
UNIT2DAY1-VDB Page 1
CH302 Vanden Bout/LaBrake Fall 2012
What are we going to learn today?
−Electromagnetic Radiation
• Understand light as an electromagnetic wave
• Understand the relationship between
frequency, wavelength, and the speed of light
−Light as Energy
• Understand how light interacts with electrons
• Explain the basic principles of the photoelectric
effect
• Recognize that light is related to frequency
• Explain the concept of a photon
CH302 Vanden Bout/LaBrake Fall 2012
What is “Light”?
Chemists use the word “light” to generally
refer to electromagnetic radiation
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CH302 Vanden Bout/LaBrake Fall 2012
Electro-Magnetic Wave
-Oscillating Electric and Magnetic Field
CH302 Vanden Bout/LaBrake Fall 2012
What is an Electric Field?
-Electric Fields surround charged particles
(and time varying magnetic fields)
-Cause charge particles to feel a force
-A fairly simply field exists between to
plates of opposite electrical charge
CH302 Vanden Bout/LaBrake Fall 2012
POLLING: CLICKER QUESTION 1
If I place an electron between these two
plates it will feel a force in what direction?
A. LeftB. RightC. UpD. DownE. None
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CH302 Vanden Bout/LaBrake Fall 2012
POLLING: CLICKER QUESTION 1
If I place an electron between these two
plates it will feel a force in what direction?
A. LeftB. RightC. UpD. DownE. None
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e-
CH302 Vanden Bout/LaBrake Fall 2012
E-Field from Light
The field is “oscillating”
http://www.enzim.hu/~szia/cddemo/edemo2.htm
CH302 Vanden Bout/LaBrake Fall 2012
Describing the waveDistance between the peaks is the wavelength
It really is a “distance”
If I am in one place, “how often” do the peaks pass?
It depends on the speed and the wavelength
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CH302 Vanden Bout/LaBrake Fall 2012
Describing the waveDistance between the peaks is the wavelength
It really is a “distance”
If I am in one place, “how often” do the peaks pass?
It depends on the speed and the wavelength
CH302 Vanden Bout/LaBrake Fall 2012
3 key parameters for a wave
Wavelength
Distance between peaks. Λ
Speed (of light)
The speed of light (in a vacuum) is constant
All light waves travel at the same speed
c = 2.998 x 108 m s-1
Frequency
The time it takes between two peaks
n = c/λ (distance s-1/distance) = s-1 (Hz)
CH302 Vanden Bout/LaBrake Fall 2012
Key Relationship
You can now do most of HW 04
CH302 Vanden Bout/LaBrake Fall 2012
Wavelengths of LightWe typically classify light by wavelength
But frequency works equally well
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CH302 Vanden Bout/LaBrake Fall 2012
Wavelengths of LightWe typically classify light by wavelength
But frequency works equally well
CH302 Vanden Bout/LaBrake Fall 2012
POLLING: CLICKER QUESTION 2
Light and Electrons
Everything has electronsIn metals those electrons can move (conductor)
What will happen if we shine light on a piece of metal?
A. The electrons will do nothing
B. The electrons will oscillate back and forth
C. The electrons will feel a force but not move
D. The electrons will turn into protons
CH302 Vanden Bout/LaBrake Fall 2012
Light and Electrons
POLLING: CLICKER QUESTION 3
What will happen if we shine brighter light?Bright light = Bigger Amplitude
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CH302 Vanden Bout/LaBrake Fall 2012
Light and Electrons
POLLING: CLICKER QUESTION 3
What will happen if we shine brighter light?Bright light = Bigger Amplitude
CH302 Vanden Bout/LaBrake Fall 2012
Light and Electrons
POLLING: CLICKER QUESTION 3
What will happen if we shine brighter light?Bright light = Bigger Amplitude
A. they will oscillate faster
B. they will oscillate with a bigger amplitude
C. more of them will oscillate
D. more of them will oscillate faster
CH302 Vanden Bout/LaBrake Fall 2012
Let’s Do an Experiment
Light + Metal Virtual Demo
http://phet.colorado.edu/en/simulation/photoelectric
If you have a laptop and want to work along with us, go to this site:
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CH302 Vanden Bout/LaBrake Fall 2012
Let’s Do an Experiment1. Run simulator.2. Select sodium. Set intensity at 25%. Set color at blue, 455 nm.3. Press start “”4. What do you see?5. What happens when increase “intensity” (Brightness) to 50%?6. What happens when increase “intensity” to 90%?7. Reduce intensity to 25%. Set color at red, 700 nm? Observation?8. Increase intensity to 50%. Observation?9. Increase intensity to 90%. Observation?10. Go back to blue, 455 nm. Switch metal to zinc. Observation?11. Increase wavelength to IR region. Observation?12. Decrease wavelength to 280 nm. Observation?13. Decrease wavelength to 185 nm. Observation? Speed of e-?
CH302 Vanden Bout/LaBrake Fall 2012
Let’s Do an Experiment
Summarize results of experiment:
CH302 Vanden Bout/LaBrake Fall 2012
Threshold Frequency
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CH302 Vanden Bout/LaBrake Fall 2012
Threshold Frequency
CH302 Vanden Bout/LaBrake Fall 2012
A whole new idea about Energy
The energy of the light is proportional to the frequency
The energy appears to come in “packets” or “photons”One photon interacts with one electron
h is Planck’s Constant
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CH302 Vanden Bout/LaBrake Fall 2012
The work function, Φ, is the minimum E need to eject and electronIt is different for different metals
CH302 Vanden Bout/LaBrake Fall 2012
If an electron is ejected from the metal surface, where does the energy of the photon go?
a)Over coming potential energy holding e- in metalb)Into the KE of the electronc)Carried away with reflected lightd)Heate)Both a) & b)
POLLING: CLICKER QUESTION 4
CH302 Vanden Bout/LaBrake Fall 2012
CLASSIC ENERGY DIAGRAM and POTENTIAL ENERGY WELL
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CH302 Vanden Bout/LaBrake Fall 2012
CLASSIC ENERGY DIAGRAM and POTENTIAL ENERGY WELL
CH302 Vanden Bout/LaBrake Fall 2012
Which of these types of light has the highest energy photons ?
A. “Green” Light (540 nm or 5.4 x 10-7 m)B. “Red” Light (650 nm or 6.5 x 10-7 m)C. Radio waves (100 m)D. X-rays (0.5 nm or 5 x 10-10 m)E. Infrared (3 mm or 3 x 10-6 m)
POLLING: CLICKER QUESTION 4
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CH302 Vanden Bout/LaBrake Fall 2012
Exciting Electrons Demo
Add electrical energy to various elements:
Describe results:
CH302 Vanden Bout/LaBrake Fall 2012
Exciting Electrons Demo
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CH302 Vanden Bout/LaBrake Fall 2012
Exciting Electrons Demo
SUMMARIZE THE SIMILARITIES AND DIFFERENCES BETWEEN THE PHOTOELECTRIC EFFECT AND THE EMISSION SPECTRA OF EXCITED ELEMENTS
CH302 Vanden Bout/LaBrake Fall 2012
What Did We Learn Today?
Light is a wave with a frequency, speed and wavelength
The energy of light is related to the frequency in a way that light seems like a particle (one photon affects one electron)
THIS ALLOWS US TO USE LIGHT TO PROBE THE ENERGY OF ELECTRONS IN MATTER
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CH302 Vanden Bout/LaBrake Fall 2012
Learning Outcomes
Understand and perform quantitative calculations based on the relationship between wavelength, energy and the speed of light.
Define wavelength, frequency, and energy of a photon.
Understand, identify, and rank the different types of light radiation.
Describe the photoelectric effect and relate the energy of a photon,the work function and the kinetic energy of the electrons, and describe the effect of the intensity and the energy of the light.
Students should understand atomic absorption and emission spectra contain discrete, very sharp lines from transition of electrons between discrete energy levels.
Apply the Rydberg formula to predict then energy of transitions between two n levels in the hydrogen atom.
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