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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


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

UNIT2DAY-LaBWednesday, September 19, 201210:25 PM

UNIT2DAY1-LaB


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


What is “Light”?

Chemists use the word “light” to generally

refer to electromagnetic radiation

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Electro-Magnetic Wave

-Oscillating Electric and Magnetic Field


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


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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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-


E-Field from Light

The field is “oscillating”

http://www.enzim.hu/~szia/cddemo/edemo2.htm

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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


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)

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Key Relationship

You can now do most of HW 04


Wavelengths of LightWe typically classify light by wavelength

But frequency works equally well

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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


Light and Electrons


What will happen if we shine brighter light?Bright light = Bigger Amplitude


Light and Electrons



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

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Light and Electrons



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


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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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-?


Let’s Do an Experiment

Summarize results of experiment:

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Threshold Frequency


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


The work function, Φ, is the minimum E need to eject and electronIt is different for different metals

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The work function, Φ, is the minimum E need to eject and electronIt is different for different metals


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)



CLASSIC ENERGY DIAGRAM and POTENTIAL ENERGY WELL

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CLASSIC ENERGY DIAGRAM and POTENTIAL ENERGY WELL


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)



Exciting Electrons Demo

Add electrical energy to various elements:

Describe results:

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SUMMARIZE THE SIMILARITIES AND DIFFERENCES BETWEEN THE PHOTOELECTRIC EFFECT AND THE EMISSION SPECTRA OF EXCITED ELEMENTS


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

UNIT2DAY1-LaB


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


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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