2 Blackbody Radiation Curve All objects emit EM radiationUsually consists of a continuous distribution of λAs temp increases the more the max intensity shifts to shorter λAs temp increases the total energy given off by the body (area under curve) increases
3 Blackbody Radiation Curve Classical physics predicts that as λ approaches 0 the amount of energy radiated should become infiniteInstead data shows that as λ approaches 0 the amount of energy radiated also approaches 01900 Max Planck developed a formula for blackbody radiation that matched the experimental data
4 Blackbody RadiationBlackbody is a hollow object with a small opening through which light can enterEnergy gets absorbed every time light hits the wall insideIf box is insulated the energy absorbed causes the temp inside to riseEnergy will be radiated inside the box and some will escape
5 Planck’s TheoryPlanck said a blackbody is made up of a large number of atomic oscillators, called resonators, which emit EM radiation of various frequenciesResonators can only absorb and give off energy in discrete amounts given byE = nhfwhere n=quantum # (0,1,2…), f=frequency of resonator, h=Planck’s constant = 6.63 x Js
6 Planck’s Theory Energy is quantized Discrete units of light energy are called quantaResonator will only radiate/absorb energy when it changes quantum statesThe idea was so radical that even Planck didn’t think it was realistic. It was just a mathematical model.
7 Photoelectric EffectPhenomenon where electrons are ejected from a metal plate when light of certain frequencies shines on it
8 Photoelectric Effect Classical physics says: When light waves of any f strike the metal they should eject electrons if their intensity is high enoughAt low intensity, electrons can be ejected if wait long enough for electrons to absorb the incoming energyIncreasing the intensity increases the KE of the ejected electronsNone of these are true!
9 Photoelectric Effect Observations: No electrons are emitted if frequency is below a certain level, regardless of intensityIf frequency exceeds this threshold frequency the number of electrons emitted is proportional to the intensityKE of emitted electrons is independent of intensityKE increases as frequency increasesElectrons are emitted almost instantly, even at low intensity1905 Einstein used Planck’s quantum idea to explain the photoelectric effect
10 Einstein’s Theory Photon – discrete unit of light energy EM waves are composed of photonsEach photon has an energy, E, given by E = nhf that can be absorbed by an electron
11 Einstein’s TheoryEinstein says that if light hits a metal it can do 1 of 2 things:Photon can give its energy to an electron in the metalPhoton can do work of removing the electron from the metal
12 Einstein’s TheoryWork function – minimum amount of energy required for an electron to escape from a metal, hft, where ft is threshold frequencyIf photon has more energy than hft the rest goes into KE of ejected electron
13 Einstein’s Theoryphoton E = energy to remove electron (work fn) + max KE of ejected electronhf = hft + KEmax
14 Einstein’s Theory If f < ft then no ejected electrons KEmaxft fIf f < ft then no ejected electronsKE only depends on f not intensity (increase in intensity means more ejected electrons but at same KE)Slope is h (Planck’s constant)Einstein’s photon model of the photoelectric effect didn’t achieve much acceptance until 1923 and Arthur Compton
15 Sample ProblemThe work function for a silver surface is 4.73 eV. Find the minimum frequency that light must have in order to eject electrons from its surface.1.14 x 1015 Hz
16 Compton Effect or Shift Compton struck an electron at rest with an xray photonNoticed scattered photon has a frequency that is less than the incident frequencyDifference between the 2 frequencies depended on the angle at which the scattered photon left
17 Compton Effect or Shift Phenomenon in which a photon is scattered with a smaller frequency or larger wavelength than the incident photonThis phenomenon became the basis for the idea of wave-particle duality.