TRANSIT OF VENUS:
1761 & 1769
1874 & 1882
2004 & 2012
"We are now on the eve of the second transit of a pair, after
which there will be no other till the twenty-first century of
our era has dawned upon the earth and the June flowers
are blooming in 2004. When the last transit season occurred
the intellectual world was awakening from the slumber of
ages, and the wondrous scientific activity which has led to
our present advanced knowledge was just beginning. What
will be the state of science when the next transit season
arrives God only knows. Not even our children's children
will live to take part in the astronomy of that day. As for
ourselves, we have to do with the present . . . ".
- William Harkness, 1882
http://outreach.as.utexas.edu/venus/pictures.html
Sparks
CH301
Why should I wear sunscreen?
ELECTROMAGNETIC RADIATION UNIT 2 Day 1
LM 11/12 and HW 6 due Monday 8:45 am
What are we going to learn today?
−Electromagnetic Radiation
• Understand light as an electromagnetic wave
• Understand the relationship between
frequency, wavelength, and the speed of light
−Light as Energy
• Understand how light interacts with electrons
• Explain the basic principles of the photoelectric
effect
• Recognize that light is related to frequency
• Explain the concept of a photon
What is “Light”?
Chemists use the word “light” to generally
refer to electromagnetic radiation
Electro-Magnetic Wave
-Oscillating Electric and Magnetic Field
What is an Electric Field?
-Electric Fields surround charged particles
(and time varying magnetic fields)
-Cause charge particles to feel a force
-A fairly simple field exists between two
plates of opposite electrical charge
POLLING: CLICKER QUESTION
If I place an electron between these two
plates it will feel a force in what direction?
A. Left
B. Right
C. Up
D. Down
E. None
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e-
E-Field from Light
The field is “oscillating”
http://www.enzim.hu/~szia/cddemo/edemo2.htm
Describing the wave Distance between the peaks is the wavelength
It really is a “distance”
If I am in one place, “how often” do the peaks pass?
It depends on the speed and the wavelength
3 key parameters for a wave
Wavelength
Distance between peaks. Λ
Speed (of light)
The speed of light (in a vacuum) is constant
All light waves travel at the same speed
c = 2.998 x 108 m s-1
Frequency The time it takes between two peaks
Which graph best represents the relationship
between frequency and wavelength for
electromagnetic radiation. (Keep in mind that
the speed of light is constant.)
λ
ν
λ
ν
λ
ν
λ
ν
A B C D
CH301 Vanden Bout/LaBrake Fall
2013
3 key parameters for a wave
Wavelength
Distance between peaks. Λ
Speed (of light)
The speed of light (in a vacuum) is constant
All light waves travel at the same speed
c = 2.998 x 108 m s-1
Frequency The time it takes between two peaks
n = c/λ (distance s-1/distance) = s-1 (Hz)
Key Relationship
You can now do most of tonight’s homework
Wavelengths of Light We typically classify light by wavelength
But frequency works equally well
POLLING: CLICKER QUESTION
Light and Electrons
Everything has electrons
In metals those electrons can move (conductor)
What will happen if we shine light on a piece of metal?
A. The electrons will do nothing
B. The electrons will oscillate back and forth
C. The electrons will feel a force but not move
D. The electrons will turn into protons
Light and Electrons
POLLING: CLICKER QUESTION
What will happen if we shine brighter light?
Bright light = Bigger Amplitude
Light and Electrons
POLLING: CLICKER QUESTION 3
What will happen if we shine brighter light?
Bright light = Bigger Amplitude
A. they will oscillate faster
B. they will oscillate with a bigger
amplitude
C. more of them will oscillate
D. more of them will oscillate faster
Let’s Do an Experiment
Let’s Do an Experiment
Make a graph of the following, keeping
other variables constant:
•Frequency vs. # of electrons
•Frequency vs. speed of electrons
•Intensity vs. # electrons
•Intensity vs. speed of electrons
PHOTOELECTRIC EFFECT:
WHAT DOES IT ALL MEAN?
KE
ν
Rb K Na
ν0(Rb)
ν0(K) ν0(Na) -hν0(Rb)
-hν0(K)
-hν0(Na)
A whole new idea about Energy
The energy of the light is proportional to the frequency
The energy appears to come in “packets” or “photons”
One photon interacts with one electron
h is Planck’s Constant
THINKING ABOUT THE PHOTOELECTRIC EFFECT
WITH THE IDEA THAT LIGHT HAS PARTICLE-LIKE
CHARACTERISTICS
Electrons are attracted to the metal nuclei.
Remember there was a threshold frequency (n0)
below which no electrons are ejected.
This amount of energy is known as F, the work
function for that metal.
If an electron is ejected from the metal
surface, where does the energy of the photon
go?
a) Overcoming potential energy holding e- in metal
b) Into the KE of the electron
c) Carried away with reflected light
d) Heat
e) Both a) & b)
POLLING: CLICKER QUESTION
Describe each of your graphs utilizing these
ideas:
thinking of light as “particles”
that one photon can interact with one electron
That intensity of light is directly related to the
number of photons
Frequency vs. # of electrons
Frequency vs. speed of electrons
Intensity vs. # electrons
Intensity vs. speed of electrons
Which of these types of light has the highest energy
photons ?
A. “Green” Light (540 nm or 5.4 x 10-7 m)
B. “Red” Light (650 nm or 6.5 x 10-7 m)
C. Radio waves (100 m)
D. X-rays (0.5 nm or 5 x 10-10 m)
E. Infrared (3 mm or 3 x 10-6 m)
POLLING: CLICKER QUESTION
What Did We Learn Today?
Light is a wave with a frequency, speed and
wavelength
The energy of light is related to the
frequency in a way that light seems like a
particle (one photon affects one electron)
THIS ALLOWS US TO USE LIGHT TO
PROBE THE ENERGY OF ELECTRONS IN
MATTER
Learning Outcomes
Understand and perform quantitative calculations based on the relationship between
wavelength, energy and the speed of light.
Define wavelength, frequency, and energy of a photon.
Understand, identify, and rank the different types of light radiation.
Describe the photoelectric effect and relate the energy of a photon,
the work function and the kinetic energy of the electrons, and describe
the effect of the intensity and the energy of the light.