Physics 102 Lecture 22 Blackbody Radiation Photoelectric Effect

Physics 102: Lecture 22 Blackbody Radiation, Photoelectric Effect, Wave-Particle Duality Demo

Hour Exam 3 • Monday, April 14 • Covers – lectures through Lecture 20 (last Monday’s lecture) – homework through HW 10 – discussions through Disc 10 • Review, Sunday April 13, 3 PM, 141 LLP

These objects are just resolved Two objects are just resolved when the maximum of one is at the minimum of the other. 43

Resolving Power To see two objects distinctly, need qobjects > qmin qobjects is angle between objects and aperture: qmin is minimum angular separation that aperture can resolve: sin qmin ≈ qmin = 1. 22 l/D D d y Improve resolution by increasing qobjects or decreasing qmin 45

Resolving Power How does the maximum resolving power of your eye change when the brightness of the room is decreased. 1) Increases 2) Constant 3) Decreases When the light is low, your pupil dilates (D can increase by factor of 10!) But actual limitation is due to density of rods and cones, so you don’t notice an effect! 47

Recap. • Interference: Coherent waves – Full wavelength difference = Constructive – ½ wavelength difference = Destructive opposite! • Multiple Slits – Constructive d sin(q) = m l (m=1, 2, 3…) – Destructive d sin(q) = (m + 1/2) l 2 slit only – More slits = brighter max, darker mins • Huygens’ Principle: Each point on wave front acts as coherent source and can interfere. • Single Slit: – Destructive: w sin(q) = m l (m=1, 2, 3…) – Resolution: Max from 1 at Min from 2 50

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, …

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!

The scale of the problem Let’s say we know an object’s position to an accuracy Dx. How much does this mess up our ability to know its speed? Here’s the connection between Dx and Dv (Dp = m. Dv): That’s the “Heisenberg uncertainty principle. ” h 6. 6 10 -34 J·s

Atomic scale effects Small Dx means large Dv since Example: an electron (m = 9. 1 10 -31 kg) in an atom is confined to a region of size Dx ~ 5 10 -11 m. How fast will the electron tend to be moving? Plug in, using h = 6. 6 10 -34 to find Dv > 1. 1 106 m/sec

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! 5

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. )

Blackbody Radiation Spectrum Visible Light: ~0. 4 mm to 0. 7 mm Higher temperature: peak intensity at shorter l

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

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/l 80% correct! Red! Highest wavelength is lowest energy. Which is hotter? (1) stove burner glowing red (2) stove burner glowing orange Hotter stove emits higher-energy photons (lower wavelength = orange)

ACT: Photon A red and green laser are each rated at 2. 5 m. W. Which one produces more photons/second? 1) Red 2) Green 3) Same Red light has less energy/photon so if they both have the same total energy, red has to have more photons! 33

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. 12

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 25

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?

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?

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=hf h=6. 626 X 10 -34 Joule sec • Maximum kinetic energy of released electrons – K. E. = hf – W 0 30

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

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 • JAVA d Source of monoenergetic electrons L 2 slitsseparated by d Screen a distance L from slits 41

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? 43

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! 45

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! 46

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! Here Kitty, Kitty! Poison 46

More Nobel Prizes! • 1906 J. J. Thompson – Showing cathode rays are particles (electrons). • 1937 G. P. Thompson (JJ’s son) – Showed electrons are really waves. • Both were right! 47

Quantum Summary • Particles act as waves and waves act as particles • Physics is NOT deterministic • Observations affect the experiment – (coming soon!) 49

See you Wednesday!