Regents Review 1.A racecar initially traveling at 55 m/s applies the breaks and comes to a stop in 2.2 s. a. What is the car’s acceleration? b. How far does the car go before it stops? Given v i = 55 m/s v f = 0 m/s a = ? d = ? t = 2.2 s

Given v i = 55 m/s v f = 0 m/s a = ? d = ? t = 2.2 s v f = v i + at 0 = 55 m/s + a(2.2 s) a = -25 m/s 2 d = v i t + ½ at 2 d = 60.5 m

Aim: How can we describe the photoelectric effect? What is light?

Proof that Light is a Wave  Light can be polarized  Diffraction Young’s Experiment Young’s Experiment  However, other experiments suggest that light might be something else

Photoelectric Effect  When light falls on metal surfaces, electrons are ejected from them  These electrons are called photoelectrons

Photocell Negatively charged metal plate (zinc) photocell Light electrons

Experiment James Clerk Maxwell Designed a photocell

Applications of the Photoelectric Effect  “Electric eyes” that open doors and turn on lights automatically  Burglar Alarms  Solar Power

Wave Theory  Wave Theory cannot explain this!

Photoelectric Effect Photoelectric Effect - Light, Quantum Mechanics, Photons - PhET Photoelectric Effect - Light, Quantum Mechanics, Photons - PhET

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According to wave theory…  What should happen to the KE of the photoelectrons if you increase the light intensity? (Remember that light intensity is the energy of the light) Wave theory says that the electrons should have more KE Wave theory says that the electrons should have more KE

 There should be a time delay between the light striking the photoelectrons and the photoelectrons moving They need time to build up their own energy They need time to build up their own energy  Frequency should have nothing to do with the energy of the photoelectrons Energy is a property of the amplitude Energy is a property of the amplitude According to wave theory…

What Really Happens  Light intensity has nothing to do with the KE of the emitted photoelectrons Light intensity (energy of the wave) max KE of ejected electrons

Instead, more electrons are ejected as light intensity increases Light intensity (energy of the wave) # of ejected electrons

Time Delay  There is no time delay between the light striking the electrons and them moving The energy of the wave is not being determined by its intensity The energy of the wave is not being determined by its intensity

Frequency  Frequency affects the energy of the ejected photoelectrons The higher the frequency, the higher the energy The higher the frequency, the higher the energy  There is a minimum frequency NO electrons will be emitted below the minimum frequency NO electrons will be emitted below the minimum frequency  This minimum frequency is called the THRESHOLD FREQUENCY (f o )

frequency max KE of ejected electrons fofo