1 27 Overview Homework: 15, 17, 23, 29, 57, 59.

2 Classical vs. Quantum Classical Theories: real number variables Quantum” theories: integer variables, e.g. electron energies in an atom Quantum theories were discovered around 1900, all theories before ~1900 were Classical

3 photoelectric effect light causes electron emission (photocurrent) Ex. garage door safety beam !Not explained by Classical theories! Photelectric phet

Photoelectric Observations Cutoff frequency fo KEmax ~ f – fo KEmax is (1.6E-19C)Vstop Ex. (1.6E-19C)(6V) = 9.6E-19J f – fo = 2E15 – 5.6E14 = 1.44E15 KEmax/(f – fo) = 6.67e-34 J/(1/s)

5 classical violations observed in photoelectric effect max. KE of electrons not dependent on intensity no electrons emitted for low frequency light, no matter how bright Einstein: light is made of small bundles of wave energy that act or don’t act Termed “photons” of light

6 Planck’s Constant, h constant connecting quantum # to actual energy: h = 6.63x J·s Ex. photon energy = hf Ex. energy of 500nm photon f = c/ : E = hf = hc/ = hc/500nm Where: hc = 1240 eV·nm

7 work function of metal photon energy breaks electron bond to metal, extra energy is distributed to electron. Bond energy = W o = hfo KE max = hf - W o Ex. Wo = 4.3 eV, KEmax = 5.0 eV: hf = 5.0eV + 4.3eV = 9.3 eV = hc/E = 1240eVnm/9.3eV = 133 nm (uv light)

8 Light Emission and Absorption white light Gas absorbtion hydrogen spectra  quantum theory of electron energies (Bohr)

9 Bohr’s Hydrogen Theory stable orbits: Photon energies are differences in energies:

10 Hydrogen Example

11 27 Summary blackbody & photoelectric data led to first quantum theories many quantum effects violate our intuition photon concept for light is connected to Planck’s constant H atom gas spectra has simplest quantum fit (energy levels related to 1/n 2 )

12 Blackbody Radiation all objects radiate EM radiation emission & peak frequency increases with temperature Wien’s Law, max T = m·K radiation energy is quantized (Planck) fits with continuous radiation spectra implied infinite radiation energy at small.

Blackbody Phet Blackbody phet

Quotes Those who are not shocked when they first come across quantum theory cannot possibly have understood it. Niels Bohr. If you are not completely confused by quantum mechanics, you do not understand it. John Wheeler. It is safe to say that nobody understands quantum mechanics. Richard Feynman.

More quotes If [quantum theory] is correct, it signifies the end of physics as a science. Albert Einstein. I do not like [quantum mechanics], and I am sorry I ever had anything to do with it. Erwin Schrödinger. Quantum mechanics makes absolutely no sense. Roger Penrose.