1761 & 1769 1874 & 1882 2004 & 2012 "We are now - - PowerPoint PPT Presentation

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

• ur 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

• ur 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

• urselves, 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
• ++++++++++++++++++

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

• ther 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.