1. Refracton of the light from the glass slide. 2. Interference of vertical and horizontal polarizations of the light

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1 Warm-up Greeen light (λ=540 nm) from a laser shines through a glass slide onto a screen. A thin layer of powder (small spheres with diameter 1µm) is deposited on the glass slide. A pattern of concentric rings appears. What's happening? 1. Refracton of the light from the glass slide 2. Interference of vertical and horizontal polarizations of the light 3. Diffraction: interference of light from the edges of each dust particle 4. The pattern is the combination of many sharp shadows from each dust particle

2 A similar pattern appears when a beam from an "electron gun" passes through a thin layer of carbon atoms (graphite foil) and hit a fluorescent screen. (Deflection by a magnetic shows that the particles are negatively charged.) What can you conclude/hypothesize? 1. Green light is made up of negatively charged particles 2. The electrons have wave properties, ie. interference and diffraction 3. The beam has particles of different mass 4. The professor is playing a trick

3 When the accelerating voltage increases, each ring get's smaller. What can you conclude/hypothesize about the electrons' wavelength? 1. As the energy increases, the wavelength increases 2. As the energy increases, the wavelength decreases 3. The wavelength does not depend on energy 4. Wavelength?

4 Blackbody Radiation Finite Temperature objects "glow" Spectrum e.g. the Big Bang (now pretty cool) Classical oscillators: (normal modes)... Rayleigh/Jeans: UV catastrophe Max Planck: a blackbody emits/absorbs radiation from a set of atomic oscillators (masses on springs) E=hf Energy depends on frequency, not amplitude! S(ν) α n 3 c 3 (e hf/kt -1) h=6.626x10-34 J/Hz k=1.23x10-23 J/ K Planck's Constant

5 What are Planck's Quanta??? Einstein: describes photoelectric effect in annus mirabilis Light (λ,f) cathode collector - current depends on light intensity - electron KE depends on λ - electron KE depends on cathode material 1921 "for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect" A Quanta are photons, each with E=hf V r Electron's KE=hf-W 0 Work function Light Intensity: S avg = Energy/Area/time S avg = cε 0 (E rms ) 2 PE Effect article by Chupp S avg = (hf) x ( # quanta Area-time )

6 Solar cells: Applications of PE Effect Light sensors: silicon photodiodes CCD (Charge coupled (silicon) device: T. Chupp 27 Oct 2004

7 Compton Effect E i,p i E e,p e E f,p f E i = E f + E e E i /c=h/λ i p i = p f + p e mv/ 1-(v 2 /c 2 ) λ f -λ i = λ C (1-cosθ) h λ c = m = 2.43x10-12 m e c maximum E e = E i /(1+m e c 2 /2E i ) demo: spectrum from 137 Cs shows photoelectric peak and Compton continuum in NaI scintillation gamma detectors. In such instruments, the photoelectron or Compton electron produces an amount of scintillatoin light proportional to the electron energy.

8 Quantum Wave Properties of Particles wave particle DUALITY λ = h p h Quantum properties important for scales λ c = m e c λ chupp 3x10-44 m Quantum Particle Properties of Light E = pc = hf particle wave Particle properties important E>1 ev (1.6x10-19 J) X-rays/Gamma rays

9 Wave Nature of Matter h : debroglie wavelength λ = p What does it "mean" Recall electron diffraction: electron intensity distributed Distribution of LOTS of electrons: Probability x Density Quantum Mechanics: Probabl ility α (Amplitude)2 recall for EM radiation: Intensity α (Amplitude)2 Electron diffraction Lecture 18 Warmup

10 A New Interpretation of Diffraction Slit of width W To reach screen, electron MUST be localized in y: Δy = W Diffraction: Δp y p sinθ = p λ W Δp y Δy λ p = h Heisenberg: Δp y Δy > h 4π Δp x Δx > h 4π ΔEΔt > h 4π Δp z Δz > h 4π conjugate pairs: (x, p x ), (y,p y ), (z,p z ), (E,t)

11 Summary: Quantum Mechanics* is 1. Weird 2. Really weird 3. Cool 4. Ask me later: We are still trying to understand it * Wave nature of particles; particle nature of light; DUALITY; debroglie waves and probability; Heisenberg's uncertainty principle Quantum Computing: quantum systems are in many states at the same time (until observerd) e.g. spins: down/up:: 0/1 makes massively parallel computing possible UM Physics Monroe Group