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

State of Late 19th 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 “Classical physics” Lecture 28 Lectures 22-25

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.

Three Early Indications of Problems with Classical Physics Blackbody radiation Photoelectric effect Wave-particle duality

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) Note humans are ‘hot’ 300K so we emit light, just not much in the visible spectrum. Try infrared.

Blackbody Radiation Spectrum Visible Light: ~0.4mm to 0.7mm Note humans are ‘hot’ 300K so we emit light, just not much in the visible spectrum. Try infrared. Classical theory at 3000 k: ultraviolet catastrophe (see p. 985 text) Higher temperature: peak intensity at shorter l Wien’s Displacement Law: lmaxT = 2.898x10-3 m·K

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/l Planck’s constant: h = 6.626 x 10-34 Joule sec Note humans are ‘hot’ 300K so we emit light, just not much in the visible spectrum. Try infrared.

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

ACT: Nobel Trivia For which work did Einstein receive the Nobel Prize? 1) Special Relativity E=mc2 2) General Relativity Gravity bends Light 3) Photoelectric Effect Photons 4) Einstein didn’t receive a Nobel prize.

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/m2) Recall: Power = Energy/time (i.e. Joules/sec.) metal light e–

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? Rate increases greater intensity increases current does not change maximum KE Nothing light e– metal

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? Nothing, but goes to 0 for f < fmin higher frequency light increases max. KE Below threshold freq, no current Electrons emitted immediately, no delay as “energy is accumulated” Increases light No e– e– e– metal

Photoelectric Effect Summary Each metal has “Work Function” (W0) which is the minimum energy needed to free electron from atom. Light comes in packets called Photons E = h f h = 6.626 x 10-34 Joule sec Maximum kinetic energy of released electrons K.E. = hf – W0 All puzzles explained with quantum theory. hf W0 KE e–

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

Quantum Physics and the Wave-Particle Duality I 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

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!

Young’s Double Slit w/ electron 2 slits-separated by d Jönsson – 1961 Go to physics 2000 web site for JAVA version Source of monoenergetic electrons L Screen a distance L from slits

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?

Young’s Double Slit w/ electron One electron at a time d Merli – 1974 Tonomura – 1989 Go to physics 2000 web site for JAVA version Source of monoenergetic electrons L Interference pattern = probability Same pattern for photons

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

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!

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

More Nobel Prizes! 1906 J.J. Thompson 1937 G.P. Thompson (JJ’s son) Showing cathode rays are particles (electrons). 1937 G.P. Thompson (JJ’s son) Showed electrons are really waves. Both were right!

Quantum Summary Particles act as waves and waves act as particles Physics is NOT deterministic Observations affect the experiment