Physics 102: Lecture 22, Slide 1 Blackbody Radiation Photoelectric Effect Wave-Particle Duality Today’s Lecture will cover textbook sections , 28.1 Physics 102: Lecture 22

Physics 102: Lecture 22, Slide 2 Everything comes unglued The predictions of “classical physics” (Newton’s laws and Maxwell’s equations) are sometimes completely, utterly WRONG. –classical physics says that an atom’s electrons should fall into the nucleus and STAY THERE. No chemistry, no biology can happen. –classical physics says that toaster coils radiate an infinite amount of energy: radio waves, visible light, X-rays, gamma rays,…

Physics 102: Lecture 22, Slide 3 The source of the problem It’s not possible, even “in theory” to know everything about a physical system. –knowing the approximate position of a particle corrupts our ability to know its precise velocity (“Heisenberg uncertainty principle”) Particles exhibit wave-like properties. –interference effects!

Physics 102: Lecture 22, Slide 4 The scale of the problem Let’s say we know an object’s position to an accuracy  x. How much does this mess up our ability to know its speed? Here’s the connection between  x and  v (  p = m  v): That’s the “Heisenberg uncertainty principle.” h  6.6  J·s

Physics 102: Lecture 22, Slide 5 Atomic scale effects Small  x means large  v since Example: an electron (m = 9.1  kg) in an atom is confined to a region of size  x ~ 5  m. How fast will the electron tend to be moving? Plug in, using h = 6.6  to find  v > 1.1  10 6 m/sec

Physics 102: Lecture 22, Slide 6 Quantum Mechanics! At very small sizes the world is VERY different! –Energy can come in discrete packets –Everything is probability; very little is absolutely certain. –Particles can seem to be in two places at same time. –Looking at something changes how it behaves.

Physics 102: Lecture 22, Slide 7 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.) Blackbody Radiation

Physics 102: Lecture 22, Slide 8 Blackbody Radiation Spectrum Visible Light: ~0.4  m to 0.7  m Higher temperature: peak intensity at shorter

Physics 102: Lecture 22, Slide 9 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/ Planck’s constant: h = X Joule sec

Physics 102: Lecture 22, Slide 10 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

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 Blue! Lowest wavelength is highest energy. E = hf = hc/ Red! Highest wavelength is lowest energy. Hotter stove emits higher-energy photons (shorter wavelength = orange)

Physics 102: Lecture 22, Slide 12 ACT: Colored Light Three light bulbs with identical filaments are manufactured with different colored glass envelopes: one is red, one is green, one is blue. When the bulbs are turned on, which bulb’s filament is hottest? 1) Red 2) Green 3) Blue 4) Same max

Physics 102: Lecture 22, Slide 13 ACT: Colored Light Which bulb’s filament is hottest? 1) Red 2) Green 3) Blue 4) Same Colored bulbs are identical on the inside – the glass is tinted to absorb all of the light, except the color you see. max Visible Light

Physics 102: Lecture 22, Slide 14 ACT: Photon A red and green laser are each rated at 2.5mW. Which one produces more photons/second? 1) Red2) Green3) Same

Physics 102: Lecture 22, Slide 15 ACT: Photon A red and green laser are each rated at 2.5mW. Which one produces more photons/second? 1) Red2) Green3) Same Red light has less energy/photon so if they both have the same total energy, red has to have more photons!

Physics 102: Lecture 22, Slide 16 Wien’s Displacement Law To calculate the peak wavelength produced at any particular temperature, use Wien’s Displacement Law: T · peak = *10 -2 m·K temperature in Kelvin!

Physics 102: Lecture 22, Slide 17 Nobel Trivia For which work did Einstein receive the Nobel Prize? 1) Special RelativityE = 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 18 Nobel Trivia For which work did Einstein receive the Nobel Prize? 1) Special RelativityE=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 19 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 20 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.) Demo

Physics 102: Lecture 22, Slide 21 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 fh= X Joule sec Maximum kinetic energy of released electrons –K.E. = hf – W 0

Physics 102: Lecture 22, Slide 22 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 23 Photoelectric Effect: Light Intensity What happens to the rate electrons are emitted when increase the brightness? –more photons/sec so more electrons are emitted. Rate goes up. What happens to max kinetic energy when increase brightness? –no change: each photon carries the same energy as long as we don’t change the color of the light

Physics 102: Lecture 22, Slide 24 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 25 Photoelectric Effect: Light Frequency What happens to rate electrons are emitted when increase the frequency of the light? –as long the number of photons/sec doesn’t change, the rate won’t change. What happens to max kinetic energy when increase the frequency of the light? –each photon carries more energy, so each electron receives more energy.

Physics 102: Lecture 22, Slide 26 Photoelectric: summary table WaveParticleResult Increase Intensity –RateIncreaseIncreaseIncrease –KEIncreaseUnchangedUnchanged Increase Frequency –RateUnchangedUnchanged Unchanged –KEUnchangedIncreaseIncrease Light is composed of particles: photons

Physics 102: Lecture 22, Slide 27 Preflights 22.4, 22.6 Which drawing of the atom is more correct? This is a drawing of an electron’s p-orbital probability distribution. At which location is the electron most likely to exist? 32 1

Physics 102: Lecture 22, Slide 28 Preflights 22.4, 22.6 Which drawing of the atom is more correct? This is a drawing of an electron’s p-orbital probability distribution. At which location is the electron most likely to exist? 32 1

Physics 102: Lecture 22, Slide 29 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 30 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 31 Young’s Double Slit w/ electron Screen a distance L from slits Source of monoenergetic electrons d 2 slits- separated by d L

Physics 102: Lecture 22, Slide 32 Young’s Double Slit w/ electron JAVA Screen a distance L from slits Source of monoenergetic electrons d 2 slits- separated by d L

Physics 102: Lecture 22, Slide 33 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 34 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 35 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 But now the interference is gone!

Physics 102: Lecture 22, Slide 36 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.

Physics 102: Lecture 22, Slide 37 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 38 Schroedinger’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 Here Kitty, Kitty!

Physics 102: Lecture 22, Slide 39 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 40 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 41 See you next lecture! Read Textbook Sections 27.5, 28.2, and 28.4