Science8 Unit C Lightand Optics Lesson5 The Wave Modelof Light
Astro&109&Lecture&9:& Lightand&Heatajbaker/ph109/midterm/Lec... · Astro&109&Lecture&9:&...
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Astro 109 Lecture 9: Light and Heat October 1, 2014
Please pick up a gra.ng (small, clear piece of plas.c)
on your way into class. This is yours to keep!
Constella=on of the day: Lyra
• Small constella=on with a big, bright star: Vega, the fiEh brightest star in the sky and (along with Altair and Deneb) defining one vertex of the “summer triangle.”
• In Greek mythology, represented the lyre of Orpheus, who unsuccessfully tried to rescue his wife Eurydice from Hades.
Image credit: IAU/Sky & Telescope
Why astronomers like Lyra
• Vega is orbited by a debris disk that represents a late stage in the forma=on of a new planetary system, in which larger pebbles, rocks, and boulders are ground into dust by collisions – likely driven by the gravita=onal tugs of planets.
Marsh et al. (2006)
Organiza=onal reminders • Midterm will be in class on Wednesday, October 15th.
• Four more sets of in-‐person office hours before the midterm: this Friday 10/3 (3:00-‐4:30pm in Scoa 102), next Tuesday 10/7 (3:00-‐4:30pm in Serin 401), next Friday 10/10, and Tuesday 10/14. Please stop by (or send me an email to make an appointment) if you need to talk!
• Reading for Friday: T7, T8, T9 (Tools of Science boxes for chapters 7, 8, and 9).
• Homework # 4 due this Friday in Sakai. (Your clicker and homework scores
should be showing up in the Sakai gradebook. I will have to make a few small adjustments for clicker ques=ons that Sakai doesn’t grade correctly.)
• G. Luzzi: please iden=fy yourself aEer class!
1/3 of class is aaending – how about you? LA DAY TIME CAMPUS ROOM
Elsie Lee Monday 1:10-‐2:30pm College Ave Kreeger 117
Isabel Kennedy Monday 6:40-‐8:00pm Livingston Tillea 111
Ryan DeGregorio Tuesday 8:10-‐9:30pm Busch ARC 326
Adrian Casper Wednesday 1:10-‐2:30pm College Ave Kreeger 117
Zac Csorny Wednesday 2:50-‐4:10pm College Ave Kreeger 117
Ragen Patel Thursday 2:50-‐4:10pm College Ave Kreeger 117
Anthony Xing Thursday 5:00-‐6:20pm Livingston Tillea 111
• All students are welcome to aaend; if you are gerng 6/10 or less on homeworks, you are strongly encouraged to aaend.
Sign up for study groups in Sakai • Wednesday/Thursday this week and Monday/Tuesday
next week are now available for signup via the “Sign-‐up” tab in Sakai (note: Sec=on 4 for today is already full).
• Regarding extra credit: – If you want extra credit, please show up on =me and par=cipate ac=vely.
– You can see your current total in Sakai gradebook in “Study group extra credit” field.
• Ques=ons from last year’s midterm you should be able to
answer aEer today’s lecture: #1 (62%), #12 (37% -‐ now you will have seen c = λν), #30 (37% -‐ caught up with material).
Finishing up on gravity…
Sept. 27 ar=st’s concep=on
Tidal forces can be destruc.ve, too • If =dal force on a moon is stronger than gravita=onal force of the moon on itself, the moon will disintegrate!
Another niEy effect of gravity: resonances • Some physical systems have “natural” frequencies where a modest input can have a drama=c effect (e.g., wine glass shaaered by opera singer’s voice). These are resonances.
• Another example: repeatedly pushing a child on a swing at the swing’s “natural” frequency yields huge amplitudes.
Image credit: www.lilsugar.com
Gravity at work: orbital resonances • Galilean moons, again: in the =me it takes Ganymede to make one orbit of Jupiter, Europa makes exactly two orbits and Io makes exactly four orbits of Jupiter.
• Coincidence? No! • Io, Europa, and Ganymede have a stable orbital resonance, which is maintained by the regular gravita=onal tugs they give each other as they orit the planet. (Not all resonances are stable, however.) – This is a liale like three children who are all pushing each others’ swings – doesn’t work on a playground, but can work with gravity.
Orbital resonances • Watch the Galilean moons do their dance (note: they never all line up on the same side of the planet)…
Image credit: Wikipedia
Extreme orbital resonances • In the =me it takes Neptune to make three orbits of the
Sun, Pluto makes exactly two orbits of the Sun.
• Since Neptune and Pluto return to the same posi=ons every 3 x 165 = 2 x 248 years, they stay far away from each other, and Pluto is protected from bullying by Neptune. – If not for the resonance, Pluto would have been kicked out of its orbit by Neptune a long =me ago…
Image credit: D. Gary
Saturn’s moons Mimas and Tethys have orbital periods of 0.94 and 1.88 days, respec=vely, defining a stable 4:2 orbital resonance. Which statement about these moons is correct?
Group clicker ques=on # 1
A. An observer on Mimas would always see the same face of Tethys at all =mes.
B. Mimas completes a single orbit of Saturn during the =me it takes Tethys to complete two orbits of Saturn.
C. Mimas and Tethys will never be on the same side of Saturn at the same =me.
D. Mimas will only pass between Tethys and Saturn once in every two of its orbits around Saturn.
E. Once Mimas is fully opera=onal, the Galac=c Empire will use it to destroy Tethys.
Image credit: NASA/Cassini
Today’s (main) subject: light and heat!
General rule: where there’s heat, there’s light of some sort.
Speed of light: first es=mate
c = 3⇥ 108 m/shap://www.ux1.eiu.edu/~cfadd/1150/01Intro/std.html
Danish astronomy Ole Rømer (1675): =me between eclipses of Io by Jupiter is longer when Earth is moving away. Right idea, right conclusion (light has a finite speed), but wrong answer; his es=mate was 25% below true value:
c stands for cosmic speed limit!
In a vacuum (i.e., empty space): + Nothing moves faster than c. + Speed of light is independent
of wavelength. (There’s no speed “bonus” if you happen to be a very energe=c photon!)
In a transparent medium (air, water, glass, etc.): + Speed of light is less than c. + Speed of light can some=mes
depend on wavelength.
Image credit: abc3miscellany.blogspot.com
The visible spectrum
Oct. 2
A prism splits light into its cons=tuent colors by taking advantage of the varia=on of the speed of light with wavelength in glass. (Your gra=ng has a similar effect for different reasons.) Roy G. Biv = red, orange, yellow, green, blue, indigo, violet – from long to short wavelength.
The electromagne.c spectrum
Light is more than meets the eye!– or at least, more than the eye can see. Astronomers now have telescopes at all these wavelengths.
Oct. 2
Demonstra=ons!
Light acts like a par=cle… and a wave
Wikipedia
Diffrac=on
aperture
Proper=es of waves
frequency, ν (cycles per second, s-‐1, or Hz)
� ⇥ = c (speed)
Why “electromagne=c”?
hap://www.nrc-‐cnrc.gc.ca/eng/projects/inms/op=cal-‐comb.html
Green light has a wavelength of around 500 nm (or 500 billionths of a meter). What is its frequency?
Example
� = 500⇥ 10�9 m
= 5⇥ 10�7 m
� ⇥ = c
⇥ =c
�
=3⇥ 108 m/s
5⇥ 10�7 m
=30⇥ 107
5⇥ 10�7s�1
= 6⇥ 1014 Hz
WRSU-‐FM broadcasts at 88.7 MHz. What is the wavelength of the radio waves?
Example
⇥ = 88.7 MHz= 88.7⇥ 106 s�1
= 8.87⇥ 107 s�1
� =c
⇥
=3⇥ 108 m/s
8.87⇥ 107 s�1
= 3.4 m
The frequency of red light is about half the frequency of violet light. In vacuum, how does the wavelength of red light compare with the wavelength of violet light?
A. four =mes larger B. twice as large C. the same D. half as large E. one quarter as large
Clicker ques=on # 2
The frequency of red light is about half the frequency of violet light. In vacuum, how does the speed of red light compare with the speed of violet light?
A. four =mes larger B. twice as large C. the same D. half as large E. one quarter as large
Clicker ques=on # 3
Percep=on
Sound: • amplitude = loudness • frequency = pitch
Light: • amplitude = brightness • frequency or wavelength = color • color and brightness are not the same!
Hot things glow: thermal radia=on
Oct. 2
Wikipedia, “thermography”
…and so do not-‐so-‐hot things
Blackbody spectrum
TF =95TC + 32
TC =59(TF � 32)
TK = TC + 273Oct. 2
Idealized object that is opaque and nonreflec=ve; need not be black per se!
Demonstra=on!
(Note: gra=ng is a giE of the Department of Physics and Astronomy… don’t use it to try to look at the Sun, but feel free to send Prof. Baker pictures to post in Sakai “art gallery”!)
As a solid body gets cooler, it emits electromagne=c radia=on
A. that is brighter and whose spectrum peaks at shorter wavelengths.
B. that is brighter and whose spectrum peaks at longer wavelengths.
C. that is fainter and whose spectrum peaks at shorter wavelengths.
D. that is fainter and whose spectrum peaks at longer wavelengths.
E. that is clammier.
Clicker ques=on # 4