Transcript Aim: How can we describe the photoelectric effect?

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
vi = 55 m/s
vf = 0 m/s
a=?
d=?
t = 2.2 s
Given
d = vi t + ½ at2
d = 60.5 m
vi = 55 m/s
vf = 0 m/s
a=?
d=?
t = 2.2 s
vf = vi + at
0 = 55 m/s + a(2.2 s)
a = -25 m/s2
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
 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
Light
Negatively
charged metal
plate (zinc)
photocell
-
+
+
+
+
+
-
electrons
-
+
Experiment
Designed a photocell
James Clerk Maxwell
1831-1879
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
 http://www.stmary.ws/highschool/physics/h
ome/animations3/modernPhysics/photoele
ctricEffect.html
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
According to wave theory…
 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
 Frequency
should have nothing to do with
the energy of the photoelectrons

Energy is a property of the amplitude
What Really Happens
 Light
intensity has nothing to do with the
KE of the emitted photoelectrons
max KE of
ejected
electrons
Light intensity
(energy of the wave)
Instead, more electrons are ejected
as light intensity increases
# of ejected
electrons
Light intensity
(energy of the wave)
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
Frequency

Frequency affects the energy of the ejected
photoelectrons


There is a minimum frequency


The higher the frequency, the higher the energy
NO electrons will be emitted below the minimum
frequency
This minimum frequency is called the
THRESHOLD FREQUENCY (fo)
max KE of
ejected
electrons
fo
frequency