The Origin and Nature of Light. Honors Project Deadlines Sorry about the confusion!! Project...

The Origin and Nature of Light

Honors Project Deadlines

Sorry about the confusion!!

•Project Proposal Deadline: Thursday March 1st 2007

•Project Submission (turn in) Deadline:

Tuesday May 1st 2007

But, what is light?• In the 17th Century, Isaac Newton argued that

light was composed of little particles while Christian Huygens suggested that light travels in the form of waves.

• In the 19th and 20th Century Maxwell, Young, Einstein and others were able to show that Light behaves both like a particle and a wave depending on how you observe it.

Thomas Young’s interference experiment

Scottish physicist James Clerk Maxwell showed mathematically in the 1860s that light must be a

combination of electric and magnetic fields.

In 1905 Einstein calculated the energy of a

particle of light (photon) and proposed the photoelectric effect.

Ephoton = hc/

e-photon

But, where does light actually come from?

Light comes from the acceleration of

charged particles (such as electrons and

protons)

electron

Accelerating charges produce light – electromagnetic radiation!

But, where does light actually come from?

Like the flavors of Ice cream – they each provide us with different information.


A

B

C

D

E

Lecture Tutorial – EM Spectrum (Handout)

• Work with a partner!• Read the instructions and questions carefully.• Discuss the concepts and your answers with

one another. Take time to understand it now!!!!

• Come to a consensus answer you both agree on.

• If you get stuck or are not sure of your answer, ask another group.

The Origin and Nature of Light• Celebration of Knowledge #2 (aka Exam #2)

is Thursday March 8th in N210

• Tailgate Party (aka exam review) is Wednesday March 7th in N210 from 4-6pm

• HW #5 – Handed out in class Feb 20th on the topic of Luminosity Area and Temperature, and Due IN-CLASS Wednesday Feb 27th


A

B

C

D

E


But what do you get when you put all the flavors (light)

together?

Luminosity is the total energy (light) emitted by an object in each second.

Stefan-Boltzmann lawLuminosity depends on an surface area (A), and its

temperature (T4)

Luminosity = 67x10)T4

Big and Hot objects have greater luminosity than

small cool objects

Lecture Tutorial – Luminosity (pg 33)

• Work with a partner!• Read the instructions and questions carefully.• Discuss the concepts and your answers with

one another. Take time to understand it now!!!!

• Come to a consensus answer you both agree on.

• If you get stuck or are not sure of your answer, ask another group.

20,000 10,000 5,000

Lum

inos

ity

(sol

ar u

nits

)

Temperature (K)

4

2

1

3

5

10,000

100

10

1

.1

.01

.001

.0001

1,000

Which star is Hot and Dim?

Temperature (K)

20,000 10,000 5,000

Lum

inos

ity

(sol

ar u

nits

)

Temperature (K)

4

2

1

3

5

10,000

100

10

1

.1

.01

.001

.0001

1,000

Which star is Cool and Dim?

Temperature (K)

20,000 10,000 5,000

Lum

inos

ity

(sol

ar u

nits

)

Temperature (K)

4

2

1

3

5

10,000

100

10

1

.1

.01

.001

.0001

1,000

Which star is Largest?

Temperature (K)

20,000 10,000 5,000

Lum

inos

ity

(sol

ar u

nits

)

Temperature (K)

4

2

1

3

5

10,000

100

10

1

.1

.01

.001

.0001

1,000

Which star is smallest?

Temperature (K)

20,000 10,000 5,000

Lum

inos

ity

(sol

ar u

nits

)

Temperature (K)

4

2

1

3

5

10,000

100

10

1

.1

.01

.001

.0001

1,000

What can we learn by analyzing starlight?

• A star’s temperature

If you pass white light through a prism, it separates into its component colors.

ROY G B I V

spectrum

long wavelengths

short wavelengths

Photographs of a Star Cluster

Spectra of a Star Cluster

Which object is hotter, an object that is emitting mainly red light or mainly blue light?

increasing temperature

Filter

Detector

81

blue 4600 A 81

Filter

Detector

85

blue 4600 A 81green 5300 A 85

Filter

Detector

83

blue 4600 A 81green 5300 A 85yellowyellow 5800 A5800 A 8383

Filter

Detector

78

blue 4600 A 81green 5300 A 85yellowyellow 5800 A5800 A 8383orange6100 A 78

Filter

Detector

70

blue 4600 A 81green 5300 A 85yellowyellow 5800 A5800 A 8383orange6100 A 78red 6600 A 70

UV IR

“Blackbody Curve” - a graph of an object’s energy output versus wavelength. The PEAK of this curve is related to the object’s temperature.

“Blackbody Curve” - a graph of an object’s energy output versus wavelength.

The WAVELENGTH that the PEAK of this curve occurs at tells us about the object’s TEMPERATURE and COLOR.

UV IR

Ene

rgy

Out

put

Wavelength

Hot objects emit light that PEAKS at short wavelengths (blue).

Cool objects emit light that PEAKS at long wavelengths (red)

increasing temperature

Wien’s law

peak = (2.9 x 10-3) / Tkelvin

• The higher the object’s temperature, the shorter the wavelength of the peak for the light emitted by the object.

Relates the temperature of an object to the wavelength of the peak in the black body curve.

What is the wavelength of the PEAK of this “Blackbody” curve

What color is our 5800K Sun?

The Sun emits all wavelengths of electromagnetic radiation (light); however, the wavelengths of light it emits most intensely are in the green/yellow part of the spectrum.

What color does the Sun appear?WHITE!!

A star, like the Sun, which peaks in the middle of the visible part of the spectrum (green/yellow light) will appear WHITE to the human eye because it is giving off nearly equal amounts of all the visible colors of light.

Our Sun

What if the Sun became hotter?

Our Sun

What if the Sun became hotter?What if the Sun became cooler?

1. Which object gives off the greatest amount of Blue light?

2. Which object gives off the greatest amount of Red light?

3. Which object would appear Red?

4. Which object would have the lowest temperature?

A

B

C

Tutorial: Blackbody Radiation (pg – 37)

• Work with a partner!• Read the instructions and questions carefully.• Discuss the concepts and your answers with one

another. Take time to understand it now!!!!• Come to a consensus answer you both agree on.• If you get stuck or are not sure of your answer, ask

another group.• We still need some volunteers to interview about the

course, the lecture-tutorials, and the online homework modules

Energy Output per second

V I B G Y O R

Star A

Star B

Wavelength

Star C

WavelengthV I B G Y O R

Star D



Star A Star A


What is this a picture of ?

Find the hottest star(s), how do you know ?

Which has the longer peak wavelength?

1. Star A2. Star C3. Same

Star C

Wavelength

V I B G Y O R


Star A

Which star has the lower surface temperature?


Star C

Wavelength

V I B G Y O R


Star A

Which of the two stars looks red?1. Star A

2. Star C

3. Both

4. Neither Star C

Wavelength

V I B G Y O R


Star A

Which star has the greater energy output?


Star C

Wavelength

V I B G Y O R


Star A

Which star is larger?


Star C

Wavelength

V I B G Y O R


Star A

Which star is larger Star A or Star D?

1. Star A2. Star D3. Same

Star D


Star A


Try to determine EVERYTHING about how these four stars compare!! Temp, Energy output, Color, size (area)…..

Object A


visible range

Ene

rgy

Out

put

per

seco

nd

Object C


visible range

Ene

rgy

Out

put

per

seco

nd

Object B


visible range

Ene

rgy

Out

put

per

seco

nd

Object D


visible range

Ene

rgy

Out

put

per

seco

nd

The Origin and Nature of Light. Honors Project Deadlines Sorry about the confusion!! Project...

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