D O N OW (2/24/14) (3 MINUTES ) What do you think Albert Einstein discovered? Is light a wave or a particle?

Wednesday we will meet in room 127! Sit in the desks when you get there.

Q UANTUM T HEORY

D ISCUSSION OF L IGHT Based on the discussion, try to come up with a list of how light acts like a wave and how light acts like a particle. DON’T try to write everything down. What type of graphic organizer can you make?

W HAT ARE SOME DIFFERENT PROPERTIES OF LIGHT ? What do we already know? Each table should write something unique on the board

I S L IGHT A W AVE ? Light ReflectsLight RefractsLight Diffracts

Dispersion of Light

(D ON ’ T WORRY ABOUT MEMORIZING THIS ) I NTERFERENCE No Interference: Single-Slit Diffraction

I NTERFERENCE Double-Slit Diffraction:

L IGHT P ROPERTIES ReflectsRefracts Diffracts and Interferes with itself

H OW DO WAVES TRAVEL ? What are some differences between light and sound? Time for a video Think about: Light vs Sound Waves vs Particles

M ICHELSON -M ORLEY EXPERIMENT VIDEO So there is no medium in space for light to travel through, maybe it’s not a wave? After watching the video, how can light travel through space if there is no Aether?

I S IT A WAVE OR A PARTICLE ? What really is the answer? edTed

I S L IGHT A PARTICLE ? How does radiation work? Light can transfer energy to objects that it collides with.

Wednesday we will meet in room 127! Sit in the desks when you get there.

E XIT T ICKET (2/24/14) Explain two examples of how light acts as a wave and two examples of how light acts as a particle.

D OUBLE S LIT Video if time _uCLwKhQ

D O N OW (2/25/14) (4 MINUTES ) What does “quantized” mean? What did we learn before that used quantization? What is the speed of light? (m/s)

Q UANTUM E NERGY What do we call particles of light? What term do we use to describe the energy of particles? Just like the charge of particles, energy is ________. Energy is ___________Energy is quantized

E INSTEIN He theorized that: Light energy is quantized Light consists of a stream of particles called photons The energy of photon (E) depends on the frequency (f) of light. E = hf h is Planck’s constant A beam of light is a stream of particles, each having the energy = hf.

P LANCK ’ S CONSTANT ( ON THE BACK PAGE ) h is Planck’s constant If energy is in Joules and wavelength is in Hz h=6.626 x J * s If energy is in eV and wavelength is in nm h*c=1240 eV*nm What is h*c? Planck’s constant * the speed of light

C OLLISIONS When a photon collides with an electron, it must either be: Reflected with no energy loss or Absorbed, transferring all its energy to the electron.

P HOTOELECTRIC E FFECT Experiment showing light is also a particle. Energy comes in particle-like chunks.

UV L IGHT When UV light is shone on a metal surface: Electrons are released instantaneously. However weak the light source is.

R ED L IGHT When red light is shone on a metal surface: No electrons are released However long light is shone on it However intense the light source is.

C LASSICAL M ECHANICS Doesn’t make sense classically. If we shine red light on a metal surface for long enough some electrons should gain sufficient energy to enable them to escape. Quantum Mechanics

C ONDITIONS FOR THE P HOTOELECTRIC E FFECT Emission of electrons only if: The frequency is above a minimum value This is called the threshold frequency f o We learned a formula today that uses frequency! If there is a minimum frequency there must be a minimum ________? Energy!

W ORK F UNCTION Electrons will only be emitted if the photons have enough energy to release them from the surface. This is called the Work Function ϕ (where ϕ = hf o ) Below this frequency no photons have enough energy to cause emission of electrons.

W AVELENGTH AND F REQUENCY What does λ stand for? Wavelength = λ What’s the relationship between f and λ? Hint: it uses f, λ, and c

UV light has a lower or higher frequency than red light, based on these wavelengths? F REQUENCY OF LIGHT Which light has a higher energy: UV or Red Light?

E MISSION OF E LECTRONS If a photon has a frequency that allow it to overcome the work function. How soon after will emission occur? Emission starts as soon as radiation falls on the surface. How fast are those photons moving? The speed of light!!!

B RIGHTNESS How is the intensity or brightness of light related to how many photons hit the metal surface? The number of electrons emitted is proportional to the brightness of the light. The brighter the light, the more photons that arrive. Brightness is ONLY dependent on number of photons.

T HE PHOTOELECTRIC E FFECT How is the Kinetic energy of the light related to the brightness of it? The kinetic energy of the emitted electrons is independent of the brightness of the light. The brighter light means more _____ Photons, but not more energetic photons.

KE OF THE EJECTED ELECTRONS Maximum Kinetic Energy of the Ejected Electrons: λ = wavelength of the incident photon λ 0 =threshold wavelength

Wednesday we will meet in room 127! Sit in the desks when you get there.

H OMEWORK Spend the rest of class working on your homework.

D O N OW (2/26/14) Sit in the desks!! What is the frequency of light that has a wavelength of 633 nm? Calculate the energy of this light in eV.

Parent teacher conferences!!! Come sign up today after school or tomorrow.

P HET L AB Each person needs to complete at least 3 problems. Initial next to each problem you solve to make sure you get credit. I should clearly see different handwritings on each paper. I will be checking the first page (front and back) at the end of class. Lab is due Friday. (If you finish early there is a bonus assignment)

C LOSING S TATEMENTS Lab is due Friday. (If you finish early there is a bonus assignment) The Link is on Ms. Timson’s BPI Page

D O N OW (2/27/14) The threshold frequency for photoelectric emission in calcium is 7.7 x Hz. Find the maximum energy in electron-volts of the ejected-electrons when light of frequency 1.2 x Hz is directed on a calcium surface.

D O N OW Q UIZ (5 MINUTES ) Clear your Desks A light of frequency 3.6 x Hz is directed on a gold surface. Find the maximum energy in electron-volts of the ejected-electrons if the threshold frequency for photoelectric emission in gold is 1.2 x Hz.

S TOPPING P OTENTIAL The stopping potential is that voltage V 0 that just stops the emission of electrons, and thus equals their original K.E. K : Maximum Kinetic Energy of the Electron q : the charge of the electron (include the negative sign) V 0 : stopping potential in V or J/C

H OMEWORK Spend the rest of class working on your homework. Photoelectric Effect or Threshold Frequency If you turn in your HW early there is a bonus assignment.

D O N OW (2/28/14) The stopping potential, V 0, that prevents electrons from flowing across a certain photocell is 5.32 V. What is the kinetic energy in J given to the electrons by the incident light?

H OMEWORK Spend the rest of class working on your homework. There will be an exit ticket at the end of class, it will be very similar to your classwork. If you finish early there is a bonus assignment.

E XIT T ICKET (2/28/14) What is the maximum wavelength of electromagnetic radiation which can eject electrons from a metal having a work function of 3.14 eV?

48 A NOTE ABOUT UNITS OF ENERGY J oules: good for macroscopic energy conversions But when talking about energy of single electrons Joules is inconvenient… (too big) Define new energy unit (the electron-volt (eV)) = kinetic energy gained by an electron when accelerate through 1 volt of potential difference E F 0V 1V path  KE = -  U = - q  V = - (- e)*(1V) = + (e)*(1V) = 1.6 x J = 1eV

49 Summary of Photoelectric experiment results. (play with sim to check and thoroughly understand) 1. Current linearly proportional to intensity. 2. Current appears with no delay. 3. Electrons only emitted if frequency of light exceeds a threshold. (same as “if wavelength short enough”). 4. Maximum energy that electrons come off with increases linearly with frequency (=1/wavelength). (Max. energy = -stopping potential) 5. Threshold frequency depends on type of metal. how do these compare with classical wave predictions?

50 Classical wave predictions vs. experimental observations Increase intensity, increase current. experiment matches Current vs voltage step at zero then flat. (flat part matches, but experiment has tail of energetic electrons, energy of which depends on color) Color light does not matter, only intensity. experiment shows strong dependence on color Takes time to heat up ⇒ current low and increases with time. experiment: electrons come out immediately, no time delay to heat up

51 Summary of what we know so far: 1.If light can kick out electron, then even smallest intensities of that light will continue to kick out electrons. KE of electrons does not depend on intensity. (Light energy must be getting concentrated/focused somehow) 2. At lower frequencies, initial KE decreases & KE changes linearly with frequency. (This concentrated energy is linearly related to frequency) 3. Is minimum frequency below which light won’t kick out electrons. (Need a certain amount of energy to free electron from metal) (Einstein) Need “photon” picture of light to explain observations: - Light comes in chunks (“particle-like”) of energy (“photon”) - a photon interacts only with single electron - Photon energy depends on frequency of light, … for lower frequencies, photon energy not enough to free an electron questions?, more sim experiments?