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

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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.

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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

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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

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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

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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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