Quantum Mechanics: Blackbody Radiation, Photoelectric Effect, Wave-Particle Duality

Transcription

1 Physics 102: Lecture 22 Quantum Mechanics: Blackbody Radiation, Photoelectric Effect, Wave-Particle Duality Physics 102: Lecture 22, Slide 1

2 opposite! Physics 102: Lecture 22, Slide 2 Recap. Interference: Coherent waves Full wavelength difference = Constructive ½ wavelength difference = Destructive Multiple Slits Constructive d sin(θ) = m λ (m=1,2,3 ) Destructive d sin(θ) = (m + 1/2) λ 2 slit only More slits = brighter max, darker mins Huygens Principle: Each point on wave front acts as coherent source and can interfere. (see Lab 8) Single Slit: Destructive: w sin(θ) = m λ (m=1,2,3 ) Resolution: Max from 1 at Min from 2

3 State of Late 19 th Century Physics Two great theories Newton s laws of mechanics, including gravity Maxwell s theory of electricity & magnetism, including propagation of electromagnetic waves But some unsettling experimental results calls into question these theories Einstein and relativity The quantum revolution Physics 102: Lecture 22, Slide 3

4 Physics 102: Lecture 22, Slide 4

5 Quantum Mechanics! At very small sizes the world is VERY different! Energy is discrete, not continuous. Everything is probability; nothing is for certain. Particles often seem to be in two places at same time. Looking at something changes how it behaves. If you aren t confused by the end of this lecture, you weren t paying attention! Physics 102: Lecture 22, Slide 5

6 Three Early Indications of Problems with Classical Physics Blackbody radiation Photoelectric effect Wave-particle duality Physics 102: Lecture 22, Slide 6

7 Blackbody Radiation Hot objects glow (toaster coils, light bulbs, the sun). As the temperature increases the color shifts from Red to Blue. The classical physics prediction was completely wrong! (It said that an infinite amount of energy should be radiated by an object at finite temperature.) Physics 102: Lecture 22, Slide 7

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9 Blackbody Radiation Spectrum Visible Light: ~0.4μm to 0.7μm Higher temperature: peak intensity at shorter λ Physics 102: Lecture 22, Slide 9

10 Blackbody Radiation: First evidence for Q.M. Max Planck found he could explain these curves if he assumed that electromagnetic energy was radiated in discrete chunks, rather than continuously. The quanta of electromagnetic energy is called the photon. Energy carried by a single photon is E = hf = hc/λ Planck s constant: h = X Joule sec Physics 102: Lecture 22, Slide 10

11 Preflights 22.1, 22.3 A series of light bulbs are colored red, yellow, and blue. Which bulb emits photons with the most energy? Blue! Lowest wavelength is highest energy. The least energy? E = hf = hc/λ Red! Highest wavelength is lowest energy. Which is hotter? (1) stove burner glowing red (2) stove burner glowing orange Physics 102: Lecture 22, Slide 11

12 Physics 102: Lecture 22, Slide 12

13 ACT: Photon A red and green laser are each rated at 2.5mW. Which one produces more photons/second? 1) Red 2) Green 3) Same Physics 102: Lecture 22, Slide 13

14 Nobel Trivia For which work did Einstein receive the Nobel Prize? 1) Special Relativity E=mc 2 2) General Relativity Gravity bends Light 3) Photoelectric Effect Photons 4) Einstein didn t receive a Nobel prize. Physics 102: Lecture 22, Slide 14

15 Photoelectric Effect Light shining on a metal can knock electrons out of atoms. Light must provide energy to overcome Coulomb attraction of electron to nucleus Light Intensity gives power/area (i.e. Watts/m 2 ) Recall: Power = Energy/time (i.e. Joules/sec.) Physics 102: Lecture 22, Slide 15

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17 Photoelectric Effect: Light Intensity What happens to the rate electrons are emitted when increase the brightness? What happens to max kinetic energy when increase brightness? Physics 102: Lecture 22, Slide 17

18 Photoelectric Effect: Light Frequency What happens to rate electrons are emitted when increase the frequency of the light? What happens to max kinetic energy when increase the frequency of the light? Physics 102: Lecture 22, Slide 18

19 Photoelectric Effect Summary Each metal has Work Function (W 0 ) which is the minimum energy needed to free electron from atom. Light comes in packets called Photons E = h f h=6.626 X Joule sec Maximum kinetic energy of released electrons K.E. = hf W 0 Physics 102: Lecture 22, Slide 19

20 Physics 102: Lecture 22, Slide 20

21 Quantum Physics and the Wave- Particle Duality I. Is Light a Wave or a Particle? Wave Electric and Magnetic fields act like waves Superposition: Interference and Diffraction Particle Photons Collision with electrons in photo-electric effect BOTH Particle AND Wave Physics 102: Lecture 22, Slide 21

22 II. Are Electrons Particles or Waves? Particles, definitely particles. You can see them. You can bounce things off them. You can put them on an electroscope. How would know if electron was a wave? Look for interference! Physics 102: Lecture 22, Slide 22

23 Young s Double Slit w/ electron JAVA d Source of monoenergetic electrons Physics 102: Lecture 22, Slide 23 2 slitsseparated by d L Screen a distance L from slits

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25 Electrons are Waves? Electrons produce interference pattern just like light waves. Need electrons to go through both slits. What if we send 1 electron at a time? Does a single electron go through both slits? Physics 102: Lecture 22, Slide 25

26 ACT: Electrons are Particles If we shine a bright light, we can see which hole the electron goes through. (1) Both Slits (2) Only 1 Slit Physics 102: Lecture 22, Slide 26

27 Electrons are Particles and Waves! Depending on the experiment electron can behave like wave (interference) particle (localized mass and charge) If we don t look, electron goes through both slits. If we do look it chooses 1. I m not kidding it s true! Physics 102: Lecture 22, Slide 27 46

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29 Schrödinger's Cat Place cat in box with some poison. If we don t look at the cat it will be both dead and alive! Poison Physics 102: Lecture 22, Slide 29

30 More Nobel Prizes! 1906 J.J. Thompson Showing cathode rays are particles (electrons) G.P. Thompson (JJ s son) Showed electrons are really waves. Both were right! Physics 102: Lecture 22, Slide 30

31 Quantum Summary Particles act as waves and waves act as particles Physics is NOT deterministic Observations affect the experiment (coming soon!) Physics 102: Lecture 22, Slide 31

32 Physics 102: Lecture 22, Slide 32