1 quiz 7: 9am +q i2i2 i1i1 i3i3. physics 7c fa 2008 quantum mechanics light as a particle energy of...
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Quiz 7:9am
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Physics 7C Fa 2008Quantum Mechanics
Light as a particle
Energy of light
Energy levels in atoms
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Outline
Quiz Introduction to quantum mechanics Extremely brief overview of major models in
the course (will be posted online if we don’t reach it)
Final Exam Logistics Course reviews
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Quiz 7:7:30am
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Quantum Mechanics Model
It can be non-intuitive We can make sense of it using two models:
Particle Wave
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Quantum Mechanics Model
Light and matter both behave like waves sometimes and like particles other times
Confining waves to an area leads to standing wave patterns For bound matter, this results in only certain
available energy levels Bound matter can gain or lose energy only
between the available levels To conserve energy, usually a photon is absorbed
or emitted with the correct energy (frequency)
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Experimental Evidence: Photons
Electrons in metal: Free to move Bound to metal
Think of as binding energy Goal: free from metal
Shine light to add energy. How can we get high energy
light?!
Laser
Metal
v
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Experimental Evidence: Photons
Binding Energy
KEelec
Light Energy
Laser
Metal
v
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Energy: Wave Model
What makes a high energy wave?a) Big wavelength
b) Big amplitude
c) High wave speed In light, what would this look like?
What do we observe if we increase the energy as the wave model suggests?
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Energy: Wave Model
High energy wave Large amplitude Large amplitude means more intensity More intensity means brighter
Result of increasing brightness: If electrons were not freed at low brightness, they
are also not freed at high brightness If electrons were freed at low brightness:
Maximum energy at high brightness is unchanged More electrons are freed
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Energy: Photon Model
Changing color of light: If red light does not free electrons, sometimes
other colors will If purple light does not free electrons, no visible
light will. Color (frequency) of light is related to the energy!
E = hf = energy of one photon h: Plank’s Constant = 6.62610-34Js
Increasing intensity corresponds to increasing number of photons.
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Which type of EM radiation is highest energy?
a) Radio b) Visible c) X-ray
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Quantum Mechanics:
Light As a wave As a particle: E=hf
Matter As a wave: =h/p As a particle: as you have always studied!
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Quantum Mechanics
Consequences of wave nature of matter 2-slit interference occurs for particles Confining a particle leads to standing waves
Different standing waves have different wavelengths Different wavelengths correspond to different
momenta Different momenta correspond to different energies
All these things are quantized: only certain values are allowed
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Atoms: electrons bound to nucleus
Molecules: can model bonds as springs, with atoms bound together
Examples of bound particles
N N
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Example of energy levels: Emission Spectra
Add energy to an element and wait. Each element emits light of certain colors. Why?
Hydrogen
Helium
Iron
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Emission Spectra
Each of the three marked transitions correspond to visible light. Which one is responsible for the red photon?
E1
E2
E3
E4
E5
a b c
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The three previous transitions corresponded to visible light. What part of the spectrum might the bold arrow correspond to?
a) Green light
b) Infrared (IR)
c) Ultraviolet
d) Microwave
e) Gamma ray
Emission Spectra
E1
E2
E3
E4E5
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The three previous transitions corresponded to visible light. What part of the spectrum might the bold arrow correspond to?
a) Green light
b) Infrared (IR)
c) Ultraviolet
d) Microwave
e) Gamma ray (way too high E)
Answers to previous 2 Qs
E1
E2
E3
E4E5
Lowest E makes red photon
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Vibrational States: An example of discrete energy The images show energy levels for two different
molecules. Which molecule can emit the longest wavelength photon, and why?
1) A: biggest ground state
2) A: highest spacing
3) C: goes up to E6
4) C: lowest ground state
5) C: smallest spacing
6) Equal: constant spacing
7) Either: both can emit photons
with arbitrarily long wavelengths.
E1 E1
E2
E2
E3
E3
E4
E4
E5
E6
A C
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Vibrational States: An example of discrete energy The images show energy levels for two different
molecules. Which molecule can emit the shortest wavelength photon, and why?
1) A: biggest ground state
2) A: highest spacing
3) C: goes up to E6
4) C: lowest ground state
5) C: smallest spacing
6) Equal: constant spacing
7) Either: both can emit photons
with arbitrarily short wavelengths.
E1 E1
E2
E2
E3
E3
E4
E4
E5
E6
A C
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Answers to previous 2 Qs Which molecule can emit the shortest
wavelength photon, and why? (7) The longest ? (4)
E1 E1
E2
E2
E3
E3
E4
E4
E5
E6
A C
4) C: smallest spacing (longest corresponds to lowest energy. We are looking for the closest together levels.)
7) Either: both can emit photons with arbitrarily short wavelengths. (shortest corresponds to highest energy. These levels continue to arbitrarily high values, so dropping from an incredibly high level to the ground level will produce a very short wavelength photon.
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Fluorescence
Observe. You will discuss more in DL
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Main Models
Wave Model Ray Model Field Model (Electricity & Magnetism) Quantum Mechanics
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Wave Model
Waves in 1, 2, and 3 dimensions Interference
1D: pathlength & phase 2D: double slit Frequency dependent: Beats Standing Waves
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Ray Model
Rays perpendicular to wavefronts Rays do not interact (i.e. no interference) Optics
Reflection Refraction Lenses (converging, diverging) Ray Tracing & Thin Lens Equations
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Field Model
Something (source) creates a field Field has magnitude at all points Field has direction at all points The net field is the vector sum of all source fields
Something else (test) interacts with the field resulting in a force Force has magnitude we can determine Force has a direction we can determine
Gravitation, Electricity, Magnetism
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Final Exam
Review sessions: probably Friday, Monday, Tuesday. Maybe some on the weekend. Check webpage for details!
Exam: When: Wednesday 3:30-5:30 Where: Giedt 1001 & Giedt 1003
Check webpage for details!
Questions?