The photon A “particle” of light A “quantum” of light energy The energy of a given photon depends on the frequency (color) of the light

But light is also a wave! Travels at constant speed c in a vacuum. c = lf c: 3 x 108m/s l: wavelength (m) f: frequency (Hz)

Calculating photon energy E = hf E: energy (J or eV) h: Planck’s constant 6.62510-34 J s or 4.14 10-15 eV s f: frequency of light (s-1, Hz)

The “electron-volt” (eV) is an energy unit Useful on the atomic level. If a moving electron is stopped by 1 V of electric potential, we say it has 1 electron-volt (or 1 eV) of kinetic energy!

Converting eV to Joules (J) 1 eV = 1.60210-19J

Photoelectric Effect experiment light light Collector (-) Photo- Diode (+) At a certain voltage, Vs, the current can’t flow anymore! e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- V e- e- A e- e- e- e- e-

Anomalous Behavior in Photoelectric Effect Voltage necessary to stop electrons is independent of intensity (brightness) of light. Photoelectrons are not released below a certain frequency, regardless of intensity of light. The release of photoelectrons is instantaneous, even in very feeble light, provided the frequency is above the cutoff.

Voltage current for different intensities of light. I3 > I2 > I1 I3 I2 Vs I1 V Stopping potential is unaffected!

Voltage versus current for different frequencies of light. f3 > f2 > f1 i f3 Vs,3 f2 Vs,2 Vs,1 f1 V Stopping potential becomes more negative at higher frequencies!

Photoelectric Effect Ephoton = Kmax + Wo hf = Kmax + Wo Ephoton = hf (Planck’s equation) Kmax: maximum kinetic energy of electrons Wo: binding energy or “work function” hf = Kmax + Wo

Graph of Photoelectric Equation hf = Kmax+ Wo Kmax = hf - Wo y = mx + b Graph of Photoelectric Equation Kmax slope = h (Planck’s Constant) Cut-off frequency f Wo (binding energy)

Absorption Spectrum DE 0 eV -10 eV Photon is absorbed and excites atom to higher quantum energy state. DE hf Ground state

Absorption Spectrum ionized 0 eV -10 eV Absorption spectra always involve atoms going up in energy level.

Emission Spectrum 0 eV -10 eV Photon is emitted and atom drops to lower quantum energy state. Excited state DE hf

Emission Spectrum ionized 0 eV -10 eV Emission spectra always involve atoms going down in energy level.

Announcements 5/6/2018 Put homework on overhead projector. Prepare for practice quiz. Next week, graded quiz and pretest distribution.

C A typical nucleus 12 6 Element name Atomic mass: protons plus neutrons C 12 6 Element name Atomic number: protons

Isotope characteristics differ U 238 92 U 235 92 Fission! Low Radioactivity

Binding energy Energy released when a nucleus is formed from protons and neutrons. Mass is lost. E = mc2 where m is the lost mass

Nuclear Particles p n Nucleons Proton 1 Neutron Charge: +e Mass: 1 amu

Nuclear reactions Nuclear Decay Fission Fusion Alpha decay Beta decay Beta Minus Positron Fission Fusion

Decay Particles Alpha Beta Positron He 4 2 e -1 e 1

Alpha Decay Rn Ra He Occurs only with very heavy elements. Nucleus too large to be stable. Rn 222 86 Ra 226 88 He 4 2

Beta Decay Occurs with elements that have too many neutrons for the nucleus to be stable. Ca 40 20 K 19 e -1 n anti- neutrino

Positron Decay Occurs with elements that have too many protons for the nucleus to be stable. H 2 1 He e n neutrino

Neutrino and Anti-Neutrino Proposed to make beta and positron decay obey conservation of energy. No mass, no charge. Energy and spin. Does not react easily with matter. Hard to detect.

Gamma Radiation, g Released by atoms which have undergone a nuclear reaction. Results when excited nuclei return to ground state. High energy! E = hf!

Fission Sr Pu n Ba 4 Occurs only with very heavy elements. Nucleus too large to be stable. Induced by neutrons. Sr 92 38 Pu 239 94 n 1 4 Ba 144 56

Fusion He H The largest amount of energy available. Energy produced in the sun. Fusion of light elements results in non-radioactive waste. He 2 H 1

Summary of Wave-Particle Duality Waves are particles and particles are waves

Energy Particle E = K + U Photon E = hf

Momentum Particle p = mv Photon p = h/l

Wavelength Photon l = c/f Particle l = h/p deBroglie wavelength

Compton Scattering Proof of the momentum of photons. High-energy photons collided with electrons. Conservation of momentum. Scattered photons examined to determine loss of momentum.

Davisson-Germer Experiement Verified that electrons have wave properties by proving that they diffract. Electron diffraction