Astrophysics Ch.5: Interaction of Light & MatterPhysics of Astronomy, winter week 8

• Star Date

• Plan for the last few weeks of winter quarter

• Modern physics pretest

• Overview of Astrophysics Ch.5: Light

• Modern physics: Giancoli Ch.38

• Your interim research reports

Plan for the last 3 weeks of winter quarter

Overview: Astrophysics Ch.5

Doppler shift:

Diffraction minima and Resolution

Photons as particles: Photoelectric effect, Compton effect

Electrons as waves: deBroglie and Bohr

0

v

c

Recall spectra: Blackbody continuum, emission by hot gas, absorption by cool gas

Diffraction minima and resolution

Diffraction grating spreads out light into colors: spectra

Angular distance between peaks depends on the separation d between slits and the wavelength :

d sin = m , where m is the spectral order (e.g. m=1 for lines nearest center).

Resolving power R of grating:

R Nm

Diffraction minima and resolution

0

v

c

Resolving power increases for diffraction grating with closer slits

Rayleigh criterion: two images are just resolvable when the center of the diffraction peak of one is directly over the minimum in the diffraction pattern of the other: = 1.22 /D, where D is the diameter of the aperture (e.g. lens or eye).

Photons as particles: Photoelectric effect

Photons can knock electrons out of metal, if they can overcome the binding energy to the metal, or work function .

Ephoton = KEelectron + binding energy: hf = Kmax +

Brighter light yields more electrons.Shorter wavelength light yields more energetic electrons.Even “weak” light beam of single photons can release e.

Photons as particles: Compton effect

Photons can collide with particles (e.g. electrons) and impart momentum and energy.

Can model photons as tiny particles of momentum p=E/c

Conservation of momentum and energy shows that photon wavelength increases by an amount

1 cose

h

m c

Electrons as waves: deBroglie wavelength

DeBroglie postulated that if light can be particles (E = hc/= pc) then maybe particles could behave like waves. What would be their wavelength?

h/= p = mvSolve for in terms of the mass and speed of the particle:

Davisson and Germer accidentally discovered that electrons do diffract as waves, thanks to an accident with their nickel crystal.

Electrons as waves: Bohr atom

Bohr combined Rutherford’s model of the orbiting electron with deBroglie’s hypothesis of electron wavelengths to• Find that angular momentum would be quantized in electron orbits• Derive energy levels for the H atom and H-like atoms.

Despite unanswered questions (such as how could such orbits be stable?), Bohr’s model fit observations:* Balmer spectra* Rydberg constant

Tuesday week 8: HW due Tues.2.Mar.

Tuesday week 8: Modern physics: Giancoli Ch.36-38Team 1: Ch.36.4, Resolution. #20, 65 Mary + Zita Team 2: Ch.37.11, E-mc^2. Q11 p.945, #36 p.946 Chelsea + Jared

Team 3: Ch.38.1-2: BB and PE effect. Q6 p.973, #15 p.974 Tristen + Matt Team 4: Ch.38.3-4: Compton Effect. Q19 p.973, #25, 28 Joey + Brian Team 5: Ch.38.5-6: waves/particles. #31, 37 Jenni + Erin Zita: Bohr atom and Quantum mechanics (38 & 39)

(You may copy figures electronically from the end of this lecture, since my laptop managed to read CDs today.)

Monday week 9: HW due Mon.8.Mar.

Astrophysics Ch.5: Interaction of Light and MatterTeam 1: Problem 5.4, photoelectric effectTeam 2: Problem 5.9, electrical vs gravitational forcesTeam 3: Problem 5.14, white dwarf and uncertainty principleTeam 4: Problem 5.17, Zeeman effect

Your interim research reports (due today)

*Read the five best articles you found from week 5 library search. *Summarize each article in a page or less, in your own words. Include the complete reference at the top of each summary, including author, journal, date, page, and title. For books, choose the best chapter or two, and reference the publisher and ISBN. *Write a 2-3-page interim report articulating your research question(s), and at least two different hypotheses that might address your question. How can you test these hypotheses? What calculations or experiments can you do to investigate them? Be as specific as possible. *Post this assignment here on WebX no later than 12:30 Mon.23.Feb. *Turn in a hardcopy at the start of class. Your prof will give you feedback the following week, and by the end of the quarter you should have a good Final Research Planning Report.

*Present a 10-minute informal report to classmates about your research question and hypotheses.

Team 2: Ch.37.11, E-mc^2. Q11 p.945, #36 p.946 Chelsea + Jared

Team 3: Ch.38.1-2: BB and PE effect. Q6 p.973, #15 p.974 Tristen + Matt

Team 4: Ch.38.3-4: Compton Effect. Q19 p.973, #25, 28 Joey + Brian

38.8- atomic models

38.9: Atomic spectra

38.10: Bohr atom