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 State of Late 19 th Century Physics Two great theories Classical physics 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 Lecture 28 Lectures Physics 102: Lecture 22, Slide 2

3 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. Physics 102: Lecture 22, Slide 3

4 Three Early Indications of Problems with Classical Physics Blackbody radiation Photoelectric effect Wave-particle duality Physics 102: Lecture 22, Slide 4

5 Blackbody Radiation Hot objects glow (toaster coils, light bulbs, the sun). As the temperature increases the color shifts from Red (700 nm) to Blue (400 nm) 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 5

6 Blackbody Radiation Spectrum Visible Light: ~0.4μm to 0.7μm Higher temperature: peak intensity at shorter λ Physics 102: Lecture 22, Slide 6 Wien s Displacement Law: λ max T = 2.898x10-3 m K

7 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 7

8 Preflights 22.1, 22.3 A series of light bulbs are colored red, yellow, and blue. Which bulb emits photons with the most energy? The least energy? Which is hotter? (1) stove burner glowing red (2) stove burner glowing orange Physics 102: Lecture 22, Slide 8

9 ACT: 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 9

10 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.) light e Physics 102: Lecture 22, Slide 10 metal

11 Photoelectric Effect: Light Intensity What happens to the rate electrons are emitted when increase the brightness? Rate increases What happens to max kinetic energy when increase brightness? Nothing light e Physics 102: Lecture 22, Slide 11 metal

12 Photoelectric Effect: Light Frequency What happens to rate electrons are emitted when increase the frequency of the light? Nothing, but goes to 0 for f < f min What happens to max kinetic energy when increase the frequency of the light? Increases light No e e e Physics 102: Lecture 22, Slide 12 metal

13 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 = x Joule sec Maximum kinetic energy of released electrons K.E. = hf W 0 hf KE e W 0 Physics 102: Lecture 22, Slide 13

14 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 14

15 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 (blackbody radiation) Collision with electrons in photo-electric effect BOTH Particle AND Wave Physics 102: Lecture 22, Slide 15

16 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 16

17 Young s Double Slit w/ electron Jönsson 1961 d Source of monoenergetic electrons Physics 102: Lecture 22, Slide 17 2 slitsseparated by d L Screen a distance L from slits

18 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 18

19 Young s Double Slit w/ electron One electron at a time Merli 1974 Tonomura 1989 d Source of monoenergetic electrons L Interference pattern = probability Physics 102: Lecture 22, Slide 19 Same pattern for photons

20 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 20

21 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 21

22 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 22

23 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 23

24 Quantum Summary Particles act as waves and waves act as particles Physics is NOT deterministic Observations affect the experiment Physics 102: Lecture 22, Slide 24