Homework No more homework!

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1 Homework No more homework! Paul Outerbridge 1 Reading Chapter 15 Light in Modern Physics Final Exam May 17 1:30-3:30 Format: like previous exams ~ 0 questions Content: ~ 50% new material 50% old material 3 4 1

2 Exam Review Question #1 Light traveling in vacuum comes to a plate of glass. Upon entering the glass, does the frequency of the light: 1. Increase. Decrease 3. Stay the same Exam Review Question # Blue light bends more than red light when entering glass from air because: 1. Red light travels faster than blue light in glass 1. Blue light travels fasters than red light in glass. Red light travels faster than blue light in vacuum 3. Blue light travels faster than red light in vacuum. 5 6 Exam Review Question #3 In what direction can you see a rainbow early in the morning? 1. North. South 3. East Properties of Light wavelength period frequency energy E = h ν power intensity frequency Planck s constant = 6.66 X10-34 m kg / s 4. West 7 8

3 What is Energy? Power the ability to do work work might be lifting an object, or causing an object to accelerate Measured in units of Joules (kg m /s ) 1 Joule ~ energy to lift an apple one meter Energy of car moving at 60 mph ~ 500,000 Joules Energy per unit time 1 Watt = 1 Joule/second Resting person ~ 80 Watts Lance Armstrong climb of L Alpe d Huez Estimated 500 Watts for 38 mins. 1 Calorie(food) = energy to raise 1 kg of water by 1 deg. C 1 Calorie(food) = 4186 Joules 9 10 Horsepower Optical Power 1 HP = 746 Watts Incandescent light Specified in electrical power 100 W bulb ~ 93 W heat; 7 W light 5 hp = 167,850 W = 168 kw Laser Pointer Specified in optical power 5 mw optical power

Intensity power/unit area measured in Watts/ unit area W/cm or W/m If the source is not directed, intensity falls off as 1/R I 1 = P/4πR 1 I = P/4πR Solar Intensity

! Compare to global power consumption ~ 10 13 W 13 14 Evidence for Photons particle-nature of light 1887 Heinrich Hertz While making radio waves with a spark-gap

4 Intensity power/unit area measured in Watts/ unit area W/cm or W/m If the source is not directed, intensity falls off as 1/R I 1 = P/4πR 1 I = P/4πR Solar Intensity Solar constant ~ 1373 W/m Distance from Sun to Earth: 150 X 10 9 m Optical power of sun: P sun ~ 1373 W/m 4 π (150 X 10 9 m) ~ 4 X 10 6 W!! Compare to global power consumption ~ W Evidence for Photons particle-nature of light 1887 Heinrich Hertz While making radio waves with a spark-gap generator, noticed it worked better when illuminated with ultraviolet light Photoelectric effect 1899 Phillip Lenard (Hertz s assistant) Studied cathode rays (electrons) produced by illuminating metal foil

Photoelectric effect 1905 The Miracle Year A number of unexplainable properties: Independent of intensity of the light No electrons if the frequency of the light was below a cutoff value The

old, clerk in Patent Office published 3 papers of great importance Brownian motion Special relativity ( E = mc ) Explanation of the photoelectric effect Nobel Prize 191 "for his services to

5 Photoelectric effect 1905 The Miracle Year A number of unexplainable properties: Independent of intensity of the light No electrons if the frequency of the light was below a cutoff value The electrons were emitted immediately - no time lag High frequency light (UV) produced higher energy electrons Albert Einstein 6 yrs. old, clerk in Patent Office published 3 papers of great importance Brownian motion Special relativity ( E = mc ) Explanation of the photoelectric effect Nobel Prize 191 "for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect" In conflict with predictions of classical physics (light as waves) Photoelectric effect It takes some energy to remove an electron from a metal surface (work function) Light is quantized (photons) with energy hν The energy of an individual photon is given to an individual electron The energy of the electron is the energy of the photon minus the work function. Explains all the features

Photoelectric effect The Start of Quantum Mechanics Planck had proposed his constant h by assuming quantized light modes to explain the blackbody spectrum But it was

Photoconductors Converts light energy into electrical energy Photodectors Photodiodes CCDs Photovoltaic cells Record efficiency 3% But still more expensive than fossil

6 Photoelectric effect The Start of Quantum Mechanics Planck had proposed his constant h by assuming quantized light modes to explain the blackbody spectrum But it was considered a calculational trick Einstein s explanation of the photoelectric effect suggested there was a reality to the quantized nature of light 1 Semiconductor Photoconductors Converts light energy into electrical energy Photodectors Photodiodes CCDs Photovoltaic cells Record efficiency 3% But still more expensive than fossil fuel Photoconductors For certain materials, the light does not eject the electron, but gives it enough energy to move (conduct) The basis for the Xerox machine Chester Carlson 3 4 6

7 Xerography The first xerox 5 7

Chapter 5 Electrons In Atoms

Chapter 5 Electrons In Atoms 5.1 Revising the Atomic Model 5.2 Electron Arrangement in Atoms 5.3 Atomic Emission Spectra and the Quantum Mechanical Model 1 Copyright Pearson Education, Inc., or its affiliates.