Announcements Read Chapter 5 for today and next time. Updated grades are online, and also submitted as your “mid-term” grade. Exam #2 “Buy-back” extra credit due in class right now! Observing starting again next week, Mon- Thur, 8:30–9:45pm, on top of this building.
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Announcements
Read Chapter 5 for today and next time.
Updated grades are online, and also submitted as your “mid-term” grade.
Exam #2 “Buy-back” extra credit due in class right now!
Observing starting again next week, Mon-Thur, 8:30–9:45pm, on top of this building.
Last Time
Solar systems formed from a collapsing nebula of gas and dust, forming a rotating disk.
Mostly hydrogen/helium, planets began to coalesce, with composition set by temperature (farther out=colder, closer in=warmer).
Last TimeExtra-solar planets: very recent discovery, very difficult to see (stars bright, planets close to stars!).
Indirect methods for finding planets around other stars: based on the “wobble” they induce in the star as they both orbit around their common center.
Light is “doppler-shifted” just like the siren from a passing fire engine. We measure the shift, and infer the motion.
Last TimeAlso use the “transit” method, if we are lucky and the planet passes in front of its star from our point of view. Have yielded first “direct” measurement of light from a planet outside our solar system.
Can study atmospheres of such transiting planets. Most dry, others with evidence of water!
Most planets found like nothing in our solar system: jupiter sized planets orbiting as close as mercury!)
Last Time
they’re the easiest to find (biggest “pull” on their stars).
Need to revise the nebular theory of solar system formation to let giant planets “migrate” inwards.
Don’t know much yet about how common earth-like planets are: still too difficult to find!
At least one “super-earth” found in the “habitable zone”, where water is liquid.
Exam #2Average: 62%, a 10% improvement from last time. ☻
Adjusted grades so that 46% is minimum passing grade (60). Max: 98%.
Buy-back extra credit will be added on top of this adjusted grade.
Grades available on the course website.
Don’t remember your class ID? See me after.
The Exam Distribution
0
12.5
25.0
37.5
50.0
0-9 10-19 20-29 30-39 40-49 50-59 60-69 70-79 80-89 90-100
The exam in review
Answer key with your grades available after class.
Let’s go over the exam questions.
Improving your gradeOnly 36% of course grade “complete”. plenty of room for improvement.
Go back and do any HW problems you skipped: you’ll still get points.
Do extra credit! New extra credit opportunity coming next week.
Don’t forget the out-of-class planetarium/observing events. These should be “freebies”, like getting 100%
The Nature of Light
The Cosmic Messenger
Beyond our neighborhood
Finishing up the Solar system.
We can reach planets, moons, comets and asteroids using satellites, landers, dig the soil, etc.
Move beyond the solar system to study stars & galaxies. Much too far to visit.
How can we learn about them?
Expand our toolset. Use light!
PREVIOUSLY
Up Next!
Light
Carries information across the universe.
To understand objects we study (stars, galaxies), we want to know what they are made of.
We need to know how light and matter interact!
What is light?Light is composed of all the colors of the rainbow
there is light beyond the colors of the rainbow (your eyes just cannot see it)
What our eyes can see is called “visible light” or “optical light”
What is light?
Light can behave both as a wave and a particle depending on how you look at it
We call this the “wave/particle duality” of light
Wave Nature of Light
Sometimes light acts like an wave.
Definitions:
wavelength: distance between adjacent peaks
frequency: the number of times any piece of the rope moves up and down in a second (Hz)
HTML-anatomy of a wave
Wave Nature of Light
Light is an electromagnetic wave (EM) that travels at the speed of light.
The strength of (invisible) electric and magnetic fields is what varies
A line of electrons would wiggle if an EM wave passed by.
Units again!
The wavelength of visible light is very small.
Red light has a wavelength of roughly 670 x 10-9 m or 670 nm.
The width of a human hair is 10,000 nm!
Frequency measured in Hz (1/s).
Wavelengths of the Rainbow
Human eye sensitivity
Frequency vs. Wavelength
Because the speed of light is fixed:
the longer the wavelength, the lower the frequency
the shorter the wavelength, the higher the frequency
Particle Nature of Light
Light can also be thought of as consisting of particles, known as “photons”
photon: a “unit” of light with a specific wavelength, frequency, and energy. A localized “wave packet”
The higher the frequency, the higher the energy
Einstein (1905) won the Nobel prize for this discovery (Not relativity!)
Properties of LightColor
Depends on frequency
blue = high frequency = short wavelength
red = low frequency = long wavelength
Carries energy (heat)
Photon energy
E = h f
high frequency = high energy = blue
low frequency = low energy = red
h = Planck’s constant
Infrared: light of heat
Infrared: light of heat
Infrared: light of heat
Infrared Zoo
Propagation of LightPhotons travel in straight lines:
energy spread over larger area at larger distances
produces 1/r2 decrease in brightness: double distance - brightness decreases by 4
Which of the following is not a form of light?
A) radio wavesB) microwavesC) x-raysD) All of the above are a form
of light.E) None of the above is a form
of light
Which of the following is not a form of light?
A) radio wavesB) microwavesC) x-raysD) All of the above are a form
of light.E) None of the above is a form
of light
✪
Which of the following has the least energy?
A) radio wavesB) visible lightC) x-raysD) infrared lightE) They all have the same
energy.
Which of the following has the least energy?
A) radio wavesB) visible lightC) x-raysD) infrared lightE) They all have the same
energy.
✪
What is matter?
All matter is made up of various chemical elements such as hydrogen, helium,carbon, oxygen, neon, iron, gold, etc.
Each element made up of different atoms.
Atoms
Atoms are composed of protons, neutrons, and electrons.
protons = +electrons = -neutrons = neutral
discovered at the turn of the 19th century
Atomic TerminologyAtomic Number: # protons in a nucleus
Atomic Mass Number: # protons+neutrons
4He
Atomic Number
defines an element
Atomic Terminology
Isotopes: Same # of protons, different # of neutrons (4He, 3He)
How do light and matter interact?
Emission (matter gives off light)
Absorption (matter absorbs light)opaque: absorbs light.
Transmission (light passes through matter). Transparent: transmits light.
Reflection or Scattering (light bounces off matter).
Interaction of Light and Matter
color films over projector
Interaction of Light and MatterWe can learn about matter from the light that it emits or reflects
Example:
The leaves in a green tree must absorb all other colors
The matter in a window must be transparent to light
A red filter absorbs all other colors than red.
Thought Question:
Why is a red rose red?
A) The rose absorbs red light
B)The rose transmits red light
C)The rose emits red light
D)The rose reflects red light
Thought Question:
Why is a red rose red?
A) The rose absorbs red light
B)The rose transmits red light
C)The rose emits red light
D)The rose reflects red light
✪
Thought Question:
Why is a red rose red?
A) The rose absorbs red light
B)The rose transmits red light
C)The rose emits red light
D)The rose reflects red light
✪
But our sense of color has to do with psychology too!
What is a spectrum?
A spectrum tells you the intensity of a given wavelength of light over a range of wavelengths
Blackbody Radiation
A perfect absorber is “black”
Absorbs all light shining on it
Absorbed light (energy) heats object
Temperature increases until:
emitted energy = absorbed energy
Emitted radiation called Blackbody Radiation
The thermal radiation emitted by most objects, include stars is similar to blackbody
Blackbody Radiation
Workbook Time
Circle your wagons.
Let’s do workbook exercise: Blackbody Radiation, on page 57.
Thermal Radiation
Thought Question:
Which star is hotter?a) the blue starb) the red star
Thought Question:
Which star is hotter?a) the blue starb) the red star
✪
Blackbody Radiation Laws
Color: wavelength at which the most light is emitted
λmax = 3 x 106 / T
T in Kelvin; λmax in nanometers (1 nm = 10-9 m)
• Cooler things are redder
• Hotter things are bluer
Color indicates temperature!
Wien’s Law
A lump of lead is heated to a high temperature. Another lump of lead that is twice as large is heated to a lower temperature. Which lump of material appears bluer?
A) The cooler lump appears bluer.
B) The hotter lump appears bluer.
C) Both lumps look the same color.
D) Cannot tell which lump looks bluer.
A lump of lead is heated to a high temperature. Another lump of lead that is twice as large is heated to a lower temperature. Which lump of material appears bluer?
A) The cooler lump appears bluer.
B) The hotter lump appears bluer.
C) Both lumps look the same color.
D) Cannot tell which lump looks bluer.
✪
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