T OF VENUS 1761 & 1769 1874 & 1882 2004 & 2012sparks.cm.utexas.edu/courses/pdf/Slides/Atomic Structure Slides Day 1... · 3 key parameters for a wave Wavelength Distance between peaks.

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.