T RANSIT OF V ENUS : 1761 & 1769 1874 & 1882 2004 & 2012 "We are now on the eve of the second transit of a pair, after which there will be no other till the twenty-first century of our era has dawned upon the earth and the June flowers are blooming in 2004. When the last transit season occurred the intellectual world was awakening from the slumber of ages, and the wondrous scientific activity which has led to our present advanced knowledge was just beginning. What will be the state of science when the next transit season arrives God only knows. Not even our children's children will live to take part in the astronomy of that day. As for ourselves, we have to do with the present . . . ". - William Harkness, 1882 http://outreach.as.utexas.edu/venus/pictures.html
Sparks CH301 Why should I wear sunscreen? ELECTROMAGNETIC RADIATION UNIT 2 Day 1 LM 11/12 and HW 6 due Monday 8:45 am
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
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 simple field exists between two plates of opposite electrical charge
POLLING: CLICKER QUESTION 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
E-Field from Light The field is “ oscillating ” http://www.enzim.hu/~szia/cddemo/edemo2.htm
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
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
Which graph best represents the relationship between frequency and wavelength for electromagnetic radiation. (Keep in mind that the speed of light is constant.) ν ν ν ν λ λ λ λ A B C D
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 tonight’s homework
Wavelengths of Light We typically classify light by wavelength But frequency works equally well
POLLING: CLICKER QUESTION 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
POLLING: CLICKER QUESTION Light and Electrons What will happen if we shine brighter light? Bright light = Bigger Amplitude
POLLING: CLICKER QUESTION 3 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
Let ’ s Do an Experiment
Let ’ s Do an Experiment Make a graph of the following, keeping other variables constant: • Frequency vs. # of electrons • Frequency vs. speed of electrons • Intensity vs. # electrons • Intensity vs. speed of electrons
P HOTOELECTRIC E FFECT : W HAT DOES IT ALL MEAN ? Rb K Na KE ν 0 (Rb) ν 0 (K) ν 0 (Na) ν -h ν 0 (Rb) -h ν 0 (K) -h ν 0 (Na)
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
T HINKING ABOUT THE PHOTOELECTRIC EFFECT WITH THE IDEA THAT LIGHT HAS PARTICLE - LIKE CHARACTERISTICS Electrons are attracted to the metal nuclei. Remember there was a threshold frequency ( n 0 ) below which no electrons are ejected. This amount of energy is known as F, the work function for that metal.
POLLING: CLICKER QUESTION If an electron is ejected from the metal surface, where does the energy of the photon go? a) Overcoming 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)
Describe each of your graphs utilizing these ideas: thinking of light as “particles” that one photon can interact with one electron That intensity of light is directly related to the number of photons Frequency vs. # of electrons Frequency vs. speed of electrons Intensity vs. # electrons Intensity vs. speed of electrons
POLLING: CLICKER QUESTION 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)
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.
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