Astronomy and CosmologiesThus.6.May, week 6, Spring 2010, Zita

• Brief Q&A• Light and Spectra• Discuss Tuesday workshops• Online spectra workshop • Caper 45 & 53 (half the class do each, then compare)

• Caper 57: Black Body Radiation

• Telescopes

• Post research updates each week

Guiding Questions for Ch.5: Light

* How can we measure the speed of light?

* How do we know that light is an electromagnetic wave?

* How is an object’s temperature related to the radiation that it emits?

* How is an object’s temperature related to the ENERGY that it emits?

(What is the evidence that light has both wave and particle properties?)

* How can astronomers detect an object’s chemical composition by studying the light it emits?

* How does an object’s motion affect the light we receive from it?

Measuring the speed of light

Ole Roemer’s astronomical measurements Modern lab measurements based on

ca. 1676 Fizeau-Foucault method

Light as an electromagnetic wave

Energy = hf = hc/

Spectrum of white light

Notice the range of visual light is about 400-700 nm (4000–7000 Å)

Infrared light has wavelengths longer than 700 nm;

Ultraviolet has wavelengths shorter than 400 nm.

Spectral lines due to absorption or emission

You looked at emission spectra of gases

Insights from Tuesday workshop? Questions?

Online Spectra Lab: http://www.learner.org/teacherslab/science/light/color/spectra/

Caper Labs:A1. Half the class do p.45-48: EM spectrum of light (in small teams)

then discuss your results with other teams;

A2. Simultaneously, half the class do p.53-56: Luminosity, Temperature, Size (in small teams)

then discuss your results with other teams;

B. Groups A1 and A2 get together to discuss results

C1. Brief lecture on Blackbody radiation

C2. Everyone do p.57-60 on Blackbody Radiation (in small teams)

Energy & Power of Blackbody Radiation

Wien’s law relates and T

(m) T(K) ≈ 3 x 10-3

where = the peak wavelength and T = the object’s temperature in Kelvin

Quiz question:

Wien’s Law:

(m) T(K) ≈ 3 x 10-3

where = the peak wavelength and T = the object’s temperature in Kelvin

Stefan-Boltzman Law relates Energy flux and Temperature:

F = T4

Where F = energy flux = Power/Area,

= 5.67 x 10-8 W m-2 K-4

T = the object’s temperature in Kelvin

Quiz question:

F = energy flux = power/area = T4

= constant, T = temperature in Kelvin

Let’s find the Luminosity = Power = Energy/time for a blackbody:

2

min

4

Power Lu osityFlux

Area R

(1) Luminosity = Flux * 4R2, and we also know that (2) Flux = T4.

Plug Flux from (2) into (1) and solve for Luminosity =

Now you know how the brightness L of a star depends on its size R and its temperature T.

C2. Now you can do the CAPER lab p.57-60 on Blackbody Radiation (in small teams)

You already know about the Doppler Effect:

The wavelength, or color, is shifted for a moving source, e.g. a galaxy.

Doppler Effect causes spectral lines to shift:

http://www.astro.virginia.edu/class/oconnell/astr121/im/dopp-shifts-spectra.jpg

From Astronomy 121 by Robert O’Connell at University of Virginia

Finally, a few words about TelescopesFirst, you don’t have to take the Ch.6 online quiz on telescopes (except Penelope and Caitlin, since this is your research project).

Do take the Ch.5 quiz on light.

Two main types of telescopes:

Refracting (bending light) and Reflecting

Virtually all modern telescopes are reflecting: why?

Ground-based telescopes can do great

… especially with adaptive optics

CCDs = revolutionary light bucket brigades

Radio telescopes can see where others can’t

Research Updates

1. Post your research updates on Moodle tomorrow• What have you observed?• How has your hypothesis developed, based on

your tests?• State your best understanding concisely• Articulate your outstanding questions clearly

2. Read the Research Updates of other teams. Give them feedback.

3. Would you like a Poster-making workshop before the Science Carnival?