Physics 116. Nov 21, Session 31 De Broglie, duality, and uncertainty. R. J. Wilkes

Transcription

1 Physics 116 Session 31 De Broglie, duality, and uncertainty Nov 21, 2011 R. J. Wilkes

2 Announcements HW 6 due today Clicker scores have been updated on Webassign gradebook Exam 3 next week (Tuesday, 11/29) Usual format and procedures I ll post example questions on Wednesday as usual We ll go over the examples in class Monday 11/28 Q: in class last week, you mentioned a story about two ways to be invisible what is the title? Classic short story by Jack London, "the shadow and the flash you can read it online: enjoy. (the physics requires a bit of poetic license, but it is fun.)

3 Lecture Schedule (up to exam 3) Today 3

4 Photoelectric effect experiment Vacuum tube with metal plates + battery cathode = negative, anode = positive If wavelength is short enough, e s escape Anode attracts any free electron cathode If electrons reach anode, current thru vacuum Shine light on cathode Blue light electrons escape, current flows Electrons have some kinetic energy KE Red light no current: electrons can t escape With blue light: try reversing voltage: light Vacuum tube V Ammeter Battery Flow of electrons = current Now put negative V on anode, + on cathode Electrons need more than just escape energy! + Anode repels e back into cathode, unless it has KE > V ev Dial down negative voltage, until current just starts again Now, value of V gives us a measurement of the KE of e s work function = minimum energy needed for escape from metal surface = (energy of photon hf) (KE) V anode + 4

5 Example : work function values What is the longest wavelength light that can eject electrons from the surface of 1) Potassium E min = W 0 = hf 0 f 0 = W 0 h 2.3eV = ev s = Hz λ 0 = c = m = 538nm f 0 ( = yellow) 2) Copper 1eV = J h = ev s h = J s E min = W 0 = hf 0 (ev/hz) Notice units: electron-volt ev = energy gained by e falling through a 1 volt potential difference. ev and h are very small! Suitable for atom-sized energy calculations f 0 = W 0 h 4.7eV f 0 = ev s = Hz λ 0 = m = 263nm ( = UV) hyperphysics.phy-astr.gsu.edu/ 5

6 Unique properties of photons We know that photons carry energy and momentum (recall: radiation pressure), and travel at the speed of light Relativity tells us that m E = 0 c 2 1 v 2 c 2 v = c but E 0 m 0 = 0 So photons must be massless Any particle moving at the speed of light must have rest mass = 0 Any particle with non-zero rest mass can never reach v=c Momentum of a photon must be m 0 v m p = 0 v p 1 v 2 c 2 E = 1 v 2 c 2 m 0 c 2 = v c p = E 2 c = hf c Example: photon from He-Ne laser has 1 v 2 c 2 p = h λ = J s 623nm = J s m = kg m / s ( ) = h λ Notice: very tiny momentum per photon, which is why we don t notice quantum effects in everyday life 6

7 Wave-particle duality All this implies light behaves like a stream of particles Compton experiment (1923): light scattering (collisions) with atomic electrons Photon collisions are same as for particle with momentum p and energy E Energy lost knocking an electron out of atom is given by same equation as for particles with mass observe a longer wavelength afterward (lower E) So photons have wave and particle character simultaneously De Broglie (1923): perhaps objects known to be particles, with mass (eg electrons), act like waves also? p = h λ for photons λ = h p for electrons More on this soon 7

8 Uncertainty principle (Heisenberg, 1927) Let s look again at single slit diffraction Illuminate single slit (width W) with light that includes some small range of wavelengths ( = small range of photon momenta) We can deduce wavelength of light from locations of fringes Photons making up the diffraction pattern came from all across the slit If we make the slit narrower, we have less uncertainty about where the photon came from But then the diffraction peaks get wider: we have more uncertainty about the wavelength (momentum) of the photons Plane waves in x direction should have p y =0, but light appearing at angle θ indicates there is a y component, py Slit width represents uncertainty in y coordinate of each photon arriving at slit. Then sinθ θ = λ W p x p y tanθ θ = p y p y p x p x p y W = h = p y y = λ W = p y h / λ Heisenberg Uncertainty principle 8

9 Heisenberg principle What does this mean? p y y = h (=very small number!) If you measure position of an electron very precisely, you cannot measure its momentum very precisely You know where the electron is, but not where it is going If you measure momentum of an electron very precisely, you cannot measure its position very precisely You know where the electron is going, but not where it is now You never notice this limitation, unless you are looking at very tiny objects or effects Try calculating uncertainty limits for a baseball: Say p = 100 kg-m/s, and you measure it to +1% What is Heisenberg limit on how well you can find its position? we ll come back to this later 9

10 Back to Young s Double Slit Experiment Deep Thought for today: Is light a particle? or a wave? 10

11 Two slits - no certainty How can we explain the 2-slit experiment for photons (particles)? Use very low intensity of light, so only one photon arrives per second Use an array of photomultiplier tubes to detect single photons arriving one by one at the screen Find: expected fringe pattern builds up as photon count rises Fringe pattern = probability distribution for photon arrival locations Non-intuitive combination of wave-like and particle-like How does a photon know about the other slit? Quantum theory says: it s impossible to simultaneously observe interference (wave property) and know which slit a particular photon came through (particle) To determine which slit it went through, you must absorb the photon! We say: probability distribution is determined by the wave character of light, and its arrival (bundle of energy transferred at some specific point in space and time) is defined by its particle character Photon is both things at once: Wave-particle duality 11

12 Building up the 2-slit pattern Wave interference picture: Particle picture: what you expect: What you actually get with particles: Particles arrive as individual events but as numbers build up, we see the interference pattern forming. 2-slit pattern = probability distribution: Statistical, not determinist! 10 particles ,000 70,000 12

13 Wave or Particle? Neither/Both/Take your pick Quantum theory says: light is both Interferes like a wave, but energy transfer occurs like a particle (photon) But exactly same for actual particles like electrons and protons! How do we know this? 1. We can do the 2-slit experiment with electrons and get the same result! 2. We can build a device which detects single photons (photomultiplier tube ) based on the photoelectric effect Modern picture of fundamental interactions (Richard Feynman, 1948) Matter as we know it is made of particles called fermions Electron, proton, neutron plus other short-lived particles produced in collisions of nuclei Forces = interactions: photon transfers of energy/momentum between particles Mediated by particles called bosons: e Photon, gluon, W/Z bosons, graviton E-M Strong Nuclear Weak nucl. Gravity Force Force Force Force e Feynman Diagram: electron emits photon which hits another electron. Like charges repel! 13

14 Let s back up a bit: Subatomic discoveries ~100 years ago J. J. Thomson (1897) identifies electron: very light, negative charge E. Rutherford (1911) bounces alpha rays off gold atoms We now know: α = nucleus of helium: 2 protons + 2 neutrons Scattering experiment = model for modern particle physics Size of atoms was approximately known from chemistry He finds: scattering is off a much smaller very dense core (nucleus ) Rutherford s nuclear model of atom: dense, positively charged nucleus surrounded by negatively charged lightweight electrons Niels Bohr (1913): applies Planck/Einstein quanta to atomic spectra Atoms have fixed energy states: they cannot soak up arbitrary energy Quanta are emitted when atom jumps from high to low E state Assumed photon s energy E=hf, as Planck and Einstein suggested Simple model of electrons orbiting nucleus, and classical physics (except for quantized E) gives predictions that match results well (at least, for hydrogen spectrum) Next topics: atoms, nuclei, radioactivity, subatomic particles 14