The Origin and Nature of Light

Honors Project Deadlines Sorry about the confusion!! Project Proposal Deadline: Thursday March 1 st 2007 Project Submission (turn in) Deadline: Tuesday May 1 st 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 19 th and 20 th 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. E photon = 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

Like the flavors of Ice cream – they each provide us with different information. 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 law Luminosity depends on an surface area (A), and its temperature (T 4 ) Luminosity =  67x10   )T 4 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 Luminosity (solar units) Temperature (K) , ,000

Which star is Hot and Dim? Temperature (K) 20,000 10,000 5,000 Luminosity (solar units) Temperature (K) , ,000

Which star is Cool and Dim? Temperature (K) 20,000 10,000 5,000 Luminosity (solar units) Temperature (K) , ,000

Which star is Largest? Temperature (K) 20,000 10,000 5,000 Luminosity (solar units) Temperature (K) , ,000

Which star is smallest? Temperature (K) 20,000 10,000 5,000 Luminosity (solar units) Temperature (K) , ,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. ROYGBIVROYGBIV 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

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

Filter Detector 81 blue4600 A81

Filter Detector 85 blue4600 A81 green5300 A85

Filter Detector 83 blue4600 A81 green5300 A85 yellow5800 A83

Filter Detector 78 blue4600 A81 green5300 A85 yellow5800 A83 orange6100 A78

Filter Detector 70 blue4600 A81 green5300 A85 yellow5800 A83 orange6100 A78 red6600 A70 UVIR “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. UVIR Energy Output 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 ) / T kelvin 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?

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 Wavelength V I B G Y O R Star D Wavelength V I B G Y O R Energy Output per second Star A Energy Output per second

What is this a picture of ? Find the hottest star(s), how do you know ?

Which has the longer peak wavelength? 1. Star A 2. Star C 3. Same Star C Wavelength V I B G Y O R Energy Output per second Star A

Which star has the lower surface temperature? 1. Star A 2. Star C 3. Same Star C Wavelength V I B G Y O R Energy Output per second 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 Energy Output per second Star A

Which star has the greater energy output? 1. Star A 2. Star C 3. Same Star C Wavelength V I B G Y O R Energy Output per second Star A

Which star is larger? 1. Star A 2. Star C 3. Same Star C Wavelength V I B G Y O R Energy Output per second Star A

Which star is larger Star A or Star D? 1. Star A 2. Star D 3. Same Star D Wavelength V I B G Y O R Star A Energy Output per second

Try to determine EVERYTHING about how these four stars compare!! Temp, Energy output, Color, size (area)….. Object A Wavelength V I B G Y O R visible range Energy Output per second Object C Wavelength V I B G Y O R visible range Energy Output per second Object B Wavelength V I B G Y O R visible range Energy Output per second Object D Wavelength V I B G Y O R visible range Energy Output per second