Unit2Day1-VandenBout Wednesday, September 18, 2013 4:13 PM Vanden - PDF document

Unit2Day1-VandenBout Wednesday, September 18, 2013 4:13 PM Vanden Bout/LaBrake/Crawford CH301 Why should I wear sunscreen? ELECTROMAGNETIC RADIATION UNIT 2 Day 1 CH301 Vanden Bout/LaBrake Fall 2013 Important Information EXAM GRADES WILL BE POSTED By SATURDAY MORNING LM12 and LM13 due Tue 9AM HW4 due Tue 9AM LAUDE LM LECTURE 1 – NO POINTs EXTRA CH301 Vanden Bout/LaBrake Spring 2013 Unit2Day1-VandenBout Page 1

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 CH301 Vanden Bout/LaBrake Fall 2013 What is “ Light ” ? Chemists use the word “ light ” to generally refer to electromagnetic radiation CH301 Vanden Bout/LaBrake Fall 2013 Electro-Magnetic Wave -Oscillating Electric and Magnetic Field Unit2Day1-VandenBout Page 2

-Oscillating Electric and Magnetic Field CH301 Vanden Bout/LaBrake Fall 2013 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 CH301 Vanden Bout/LaBrake Fall 2013 Unit2Day1-VandenBout Page 3

POLLING: CLICKER QUESTION 1 If I place an electron between these two plates it will feel a force in what direction? ++++++++++++++++++ A. Left ------------------ B. Right e - C. Up D. Down E. None CH301 Vanden Bout/LaBrake Fall 2013 E-Field from Light The field is “ oscillating ” http://www.enzim.hu/~szia/cddemo/edemo2.htm CH301 Vanden Bout/LaBrake Fall 2013 Unit2Day1-VandenBout Page 4

POLLING: CLICKER QUESTION 2 If I place an electron in the presence of an oscillating electric field the electron will? A. Left B. Right e - C. Up D. Down E. Oscillate CH301 Vanden Bout/LaBrake Fall 2013 Describing the wave Distance 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 CH301 Vanden Bout/LaBrake Fall 2013 Unit2Day1-VandenBout Page 5

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 10 8 m s -1 Frequency The time it takes between two peaks n = c/ λ (distance s -1 /distance) = s -1 (Hz) CH301 Vanden Bout/LaBrake Fall 2013 Key Relationship You can now do most of HW 04 CH301 Vanden Bout/LaBrake Fall 2013 Wavelengths of Light We typically classify light by wavelength But frequency works equally well Unit2Day1-VandenBout Page 6

We typically classify light by wavelength But frequency works equally well CH301 Vanden Bout/LaBrake Fall 2013 POLLING: CLICKER QUESTION 3 Light and Electrons Everything has electrons In 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 CH301 Vanden Bout/LaBrake Fall 2013 Unit2Day1-VandenBout Page 7

POLLING: CLICKER QUESTION 4 Light and Electrons What will happen if we shine brighter light? Bright light = Bigger Amplitude CH301 Vanden Bout/LaBrake Fall 2013 POLLING: CLICKER QUESTION 4 Light and Electrons 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 CH301 Vanden Bout/LaBrake Fall 2013 Unit2Day1-VandenBout Page 8

Let ’ s Do an Experiment Light + Metal Virtual Demo Use one laptop per group and work together on the activity: http://phet.colorado.edu/en/simulation/photoelectric CH301 Vanden Bout/LaBrake Fall 2013 Let ’ s Do a Virtual Experiment Using Photoelectric Effect Simulator CH301 Vanden Bout/LaBrake Fall 2013 Let ’ s Do an Experiment Summarize results of experiment: Unit2Day1-VandenBout Page 9

’ Summarize results of experiment: CH301 Vanden Bout/LaBrake Fall 2013 Threshold Frequency CH301 Vanden Bout/LaBrake Fall 2013 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 Unit2Day1-VandenBout Page 10 ’

The energy appears to come in “ packets ” or “ photons ” One photon interacts with one electron h is Planck ’ s Constant CH301 Vanden Bout/LaBrake Fall 2013 The work function, Φ, is the minimum E need to eject and electron It is different for different metals CH301 Vanden Bout/LaBrake Fall 2013 POLLING: CLICKER QUESTION 5 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 metal b)Into the KE of the electron c)Carried away with reflected light d)Heat e)Both a) & b) CH301 Vanden Bout/LaBrake Fall 2013 Unit2Day1-VandenBout Page 11

CLASSIC ENERGY DIAGRAM and POTENTIAL ENERGY WELL CH301 Vanden Bout/LaBrake Fall 2013 POLLING: CLICKER QUESTION 6 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 m m or 3 x 10 -6 m) CH301 Vanden Bout/LaBrake Fall 2013 Unit2Day1-VandenBout Page 12

Quick Review of DNA CH301 Vanden Bout/LaBrake Fall 2013 Why Should I wear Sunscreen? TYPES of DNA DAMAGE CH301 Vanden Bout/LaBrake Fall 2013 Unit2Day1-VandenBout Page 13

Why Should you wear Sunscreen? AVOBENZONE – common active ingredient, UV max 357 nm Zinc Oxide – reflects UV light CH301 Vanden Bout/LaBrake Fall 2013 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 CH301 Vanden Bout/LaBrake Fall 2013 Unit2Day1-VandenBout Page 14

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. CH302 Vanden Bout/LaBrake Fall 2012 Unit2Day1-VandenBout Page 15